Shipped fast looks well made for what it is . Haven't been able to use waiting for colder weather .You would think for a big name brand that they'd have better quality construction. Just opened up the liner to test it out at home and there's already a big hole in the footbox.UPDATE:I originally reviewed this about a week and a half ago after receiving it in the mail. I was never contacted by Sea to Summit about the issue. Now I'm all about honest reviews and that's why I posted the review in the first place. But I also like to fix things and although super annoying and frustrating that it came with a torn seam right out of the package, I fixed it myself. Just a little basic sewing. Shout out to Youtube!I used this for the first time this past weekend. I have the Teton Tracker sleeping bag that I use in my hammock and I purchased this to go along with it. The very first night out it ended up raining and sleeting and winds were about 20mph. I had not broken out the liner yet because I wanted to push the limits of my sleeping bag and see how well it would do. About halfway through the night though I had to reach up and grab the liner off of my ridgeline because I started getting cold in my bag. I was in my sleeping bag in my hammock with an underquilt. Temps this night were dropping to the low 20s F (approx. -7 to -4 degrees C for my metric peeps) and with the windchill is was easily in the teens (-12 to -7 degrees C). My sleeping bag is rated down to 5F (-15C) but for anyone who understands sleeping bag ratings, that's what you can survive at, no what you'll be comfortable in. I also didn't have the proper underquilt for my hammock since it was rated down to 40F I believe. So needless to say, I started getting a tad bit chilly as the night went on and the temp kept dropping.This is where some other issues with this liner arise. If you've read the Q&A section on this product or any of the reviews you'll have figured out that there is only one opening into this thing. So imagine trying to unravel a 7ft (2.13m) tall 100% polyester lining thats super elastic, all while trying to stay in your hammock in your sleeping bag and out of the rain/sleet. It didn't work out too well. I ended up saying screw it and just covered up my torso with the liner. EVEN JUST COVERING UP MY TORSO I WAS SIGNIFICANTLY WARMER. Just that extra layer in my sleeping bag made quite the difference.The second and third nights the weather was still very cold, but the wind was just a slight breeze and it was clear skies. Both of these nights I actually got into the liner completely before getting into my sleeping bag in the hammock. I stood on a piece of plastic underneath my hammock (where I set my boots) and stepped into the liner, bringing it up to my armpits before actually getting into my sleeping bag in the hammock. Still not the easiest thing to do because of course the liner isn't stapled to me so it doesn't stay still.The BIGGEST drawback to this liner I believe is having just the one opening. Obviously if you're in a tent, although still inconvenient, this isn't that big of a deal. But if you're hammock camping, this is my heads up to you.BUT, aside from the hassle of getting into it and having a torn seam out of the package (still salty about it), I will say that this liner DID add significant warmth to my sleeping bag. As other reviews have said I do not think that it adds the 25F (15C) that they claim. But it does add significant warmth and I was quite toasty the second and third nights on this trip.Overall, if you're looking for liner to add some warmth to your bag it's probably safe to say that this will do the trick. Just be cautious of the three things that I have mentioned:1) Quality of the seams (inspect them as soon as you get it!)2) Pretty positive it will not add the 25F (15C) that they claim. Probably more along the lines of 10 F (give or take)3) Only one way into this thingSome quick PROS that I'll add are the packability, weight, and warmth.After sewing the seam on my own I will continue using this liner. As I said, I am happy with the warmth it added. I will post an update down the road after I get some more use out of it! But since it did keep me from freezing my but off the second two nights I'm going to add another star to my rating, making it 3 stars.The material is soft and comfortable, and it does help to warm a bit. However, it doesnt hold warm air in, so it's not very useful on its own over a blanket. It's lightweight, but if used to make a bag warmer and weight is a concern then just getting a warmer bag makes sense.I was hoping to use this alone in some high 50s low-60s overnight Temps, and it can't do that. When I hike I care about my pack weight a lot, so getting a cooler bag (vs the 35 degree rated one I have) ended up making more sense for my needs.I could see this being a good product for camping, where you're not hiking with everything on your back, and you just need to warm up a bag.Great buy. I was surprised the bright red didn't bleed out in the wash or on my skin. It was a great size both in length (I'm 6ft) and in width (enough for 2 average people to fit inside). The liner made a huge difference between 15-35f and as a stand alone in 60-70f (for when you want something but a sleeping bag is to warm).Great addition to my sleep system outdoors!I paid such a ridiculously high price for this product that I am too ashamed and embarassed to admit it, to you. I did, however, decide to purchase it, anyway, despite the high premium. Here is why ... So, I have constructed a lightweight 3-season system, for Central Texas, which does get "very" cold, at unpredictable intervals, throughout the late-fall to early-spring "trimester". Central Texas is Hill-Country ... Lots of steep ridges and canyons, all bunched together and there is lots of climbing involved and major weather fronts converge, in this region, manifesting all sorts of conditions, sporadically and unpredictably. My sleep-system consists of ONE G.I. Gortex Bivy-Bag and one SnugPak compact and lightweight "JUNGLE" bag (with integral bug-net), ... In order to augment this basic system, I also carry a VERY light and VERY compact Adventure Medical ThermoLite 2.0 Reflective Emergency Bivy AND ... YES ... This (equally ultra-light and compact) ThermoLite Reactor Extreme Mummy Liner. In colder weather, I may carry an additional silk or fleece bag liner. *** COST JUSTIFICATION *** These augmentation components fit in your open hand, and are smaller than a standdard roll of toilet-paper (and are a little lighter.) This allows for a very light, compact, and highly mobile and flexible sleep system that can "stretch" your temperature tolerance by (very hopefully) 25 to 30 degrees, F. I also carry a Short RidgeRest (aluminized) closed-cell pad, which is shorter and narrower than most pads. To be honest, I do not believe that these (ThermoLite) ultralight options perform to their optimistic claims, of 15 degrees for the thermal bivy and 25 degrees for the Reactor-Extreme bag liner ... Again, used together, properly, ?? ... 30 degrees of boost is the most one could hope for, and is probably overly optimistic ... I would expect to get slightly better than 20 degrees using BOTH thermalite components, if the temps were below freezing. Anything that weighs less than a roll of toilet-paper and is smaller, that can boost my sleep comfort (and survival envelope) by ANY significant amount MUST be worth spending a bit "more". ... And ... I sure DID ! Sorry ... no anecdotal gripes, as of yet ... I try not to use this resource any more than necessary. If I KNOW ... IN ADVANCE ... That it will be cold, then I will pack my USGI-ECWS-IMSS which is 12 pounds and will handle anything Texas can throw at me (any time).Hope this helps.I used this combined with a zero bag in Winter backpacking where it got down to 10 degrees and it was not as warm as I had hoped. It helped and I didn’t get hyperthermia, but I thought it would be warmer.This is being used as a replacement to my SOL bivy as a added layer of warmth when backpacking. It’s a long tube that you slide into and it’s very cozy. I used it in a tent on a Sea to summit air mattress with a small down sleeping bag into the low 40’s on a mountain in Virginia. It was very convenient to use. My wife used one in a hammock with her underquilt and it takes a little more effort to shimmy into for the hammock. But nothing too difficult. It works well for its purpose and in hot summer nights a will try using just it.I bought this to add an extra few degrees tolerance to my winter sleep system wild camping. It does do this though I'm not sure it's to the degree they're stating. It's certainly nice to have something to line the bag with but I do find that due to the material and the materials generally used in sleeping bags that this bag tends to slide around, scrunch up or sink to the bottom of the bag quite easily and getting both to fit snugly at the same time is much like trying to wear two condoms; sure you're better protected in theory, but in practise the shuffling around to line yourself up and not feel like your somewhat helplessly tangled up kind of deflates my excitement for the product somewhat. Still, it's nice to have. Should really be cheaper than it is though for what it is. I can't imagine it cost much to manufactureI go hiking a lot and have resisted getting a liner due to the idea I would feel constricted in my sleeping bag.Well, my fears have been put to rest. Firstly, this liner is so soft and comfy it feels like a cost sheet wrapping around you. The material is smooth and light.Importantly, and this is why I got it - the liner adds a lot of warmth to my sleeping bag. I’d say a other season. I’ve seen claims that it adds 23 degrees C in warmth. Well I certainly don’t think that to be true, however it made a noticeable difference a couple of weekends ago in 1-2 degree overnight temperatures.It has a hood with a draw string to keep head warmth in.It is however expensive in my opinion, hence 4 stars for value for money. But having paid the money for it, I am so far happy with my purchase.This is a good piece of equipment to provide a very moderate boost to warmth on a cold night. You can forget about the claimed 8°c boost, it's more like 1-3. It feels nice and warm next to the skin, wicks away perspiration, increases general comfort and helps minimise drafts. It also keeps you sleep system cleaner.This is not a very efficient way to boost the warmth of you sleep system. 0.24kg for a 1-3°c boost is weight inefficient so bare this in mind. This item can take a summer sleeping bag about half way into the autumn season. It can't do much more.Used this to bump up a 3 season sleeping bag to a 4 season for some sub-zero camping. The difference it made was certainly not equivalent to adding a "season" - it made me marginally warmer, and was soft and comfortable and is well made, but I can't imagine that it is any better than a bog standard fleecy liner! If I need 4 season warmth in future I shall be buying a 4 season sleeping bag and not relying on this for anything other than keeping the sleeping bag clean inside.I crossed the Atlantic Ocean on a small sailing boat using this.It's perfect for warm environments and even when it's a bit chilly, it offers some protection and something to get cosy inside of. Feels lovely and soft. Rolls up easily and stuffs back into it's bag fairly quickly too - important when you're a yacht racer :) Washes easily.Useful if you want to sleep and protect yourself from lying straight onto a surface with no sheet.For colder environments you'd need a blanket or much something much thick. Anything under 15 degrees C and you'd be cold I think.