This bag is perfect for summer camping in the mountains. I'm 6'3" and I fit almost perfectly in the long version. It is true what others say about just how small it gets, but to take full advantage of its packability, buy an extra small compression sack that can make it as small as a softball. It did get a little below 40 and I needed to throw on my Patagonia Down Sweater and capaline long johns, but after that I was toasty warm. I combined this with the Thermarest NeoAir for an extremely comfortable, extremely light sleeping system.

The Marmot Atom is hands-down the best 40 degree bag out there. I purchased it to replace my 25 degree bag during warm weather camping and could not be happier. Before purchasing the Atom, I used to overheat and often ended up sleeping on top of my bag. Those days are long gone! Even in considerably warm temps, I have found this bag regulates my temperature well and breathes phenomenally - I have yet to even hang a leg out of the zipper. Speaking of which, the zipper does not extend all the way to the bottom of the bag, so if you are one of the leg-hangers out there, this may be something to think about. The shell material is absolutely amazing. It is incredibly light and it feels like silk! It also does a decent job of shedding water from the bag to preserve the outstanding loft of the down. As an ultralight junkie, my favorite thing about the bag has to be its packed size and weight. I easily compress it to the size of a cantaloupe, but it could be forced even smaller without much effort given the right compression sack. Marmot does include a stuff sack, but unfortunately it does not have any compression straps. To really take advantage of this bags packability, you will need to find an extra-small compression sack elsewhere. Search around for the smallest one you can find, because this bag _will_ fit in it. I admit, this sleeping bag is a bit pricey, but it is currently the best 40 degree bag money can buy. If you can afford it and are sick of sleeping on top of your overkill sleeping bag during summer backpacking trips, the Atom is a must!

I have the previous generation of the product, as well as the new (2009) model. I bought the new one for the new, full length, two way zipper. They otherwise seem functionally the same. They are extremely lightweight, compact, and durable. I have used it on multiple occasions with temperatures in the 28-32 degree range (the 2007 model), and have generally been quite comfortable with a light base layer and thermal cap, though I think this is the lower limit of the bag. I was in it into the mid 20's once (it got alot colder than forecast) and was a little too chilly despite stuffing some of the clothes I had into the bag with me for extra insulation. The new full lenth, 2-way zipper makes a great bag even more flexible, extending the comfort range into very warm temps, since you can now "hang out a leg" or unzip all the way down. The only negative I can find is the price is a little high, but I think it's worth every penny, and highly recommend it!

I love this sleeping bag. It weighs next to nothing, and fits in a tiny stuff sack the size of a Nalgene bottle (really). The temp rating is pretty accurate - I've been cold below 40, but it's been manageable. When it gets colder, I usually double up and wear a jacket to bed as well. Way pricey, but if you're gear head like me, totally worth it. I've seen it on steepandcheap.com - much to my chagrin, not before I bought it.

Kept my pack down to forty-five pounds on a climbing trip into the Sierras this summer with the help of this lightweight, but warm bag. I was skeptical at first as it seemed so small and thin, but temps dropped down into the thirties several nights and I was plenty toasty in this well-made bag.

I purchased this bag and it is the lightest 3 season bag out there. I used this bag on in an october camping trip where temps were estimated to range in the low 40's high 30's. The temp at 5 am was 27 degrees. I was wearing long johns socks and a hat and I was perfectly warm. Awesome bag. The material feels way thin, and if you don't wear clothes it will stick to you. But other than that it's awesome.

I also purchased this bag for 3-season backpacking and I'm very pleased with the product. I have used it 6 times at different morning temperatures and the 40 degree rating is about right - if you're in a tent. I sleep reasonably warm, and when the temp was below 40 I got cold. Above 40, in a tent, I have been fine. Above 50 you will be unzipping it. I don't sleep in anything except a polypro shirt and underwear, liner socks, and a wool beanie, so if you add a layer of thermal underwear, you can decrease the minimum temp.

As SpriteMV says, the bag compresses very well. I can stuff it in the bottom compartment of my pack and still have room for my tent, footprint, and camp shoes. Try that with any other bag!

The zipper is a bit on the small side, certainly not a #10, so be careful when using it. There is a draw-string hood which works well.

Overall, if you want to shave a couple of pounds from your basic gear set for 3-season camping, the Atom is certainly the ticket.

After using countless crummy rental/borrowed synthetic bags I finally bought myself the Marmot Atom bag. While the price is a bit steep I absolutely do not regret buying it one bit. I have used it in temps down to the mid-30's and it has been perfect. It is incredibly soft and comfortable. Another benefit is that it has an extremely simple design: one zipper and one pull-cord. No jumble of zippers and cords and velcro attacking your face as you sleep like with so many other sleeping bags. The full-zip design is great for warmer weather camping as well as the dual-sided zipper for venting your feet if they get too hot. The hood is great for those colder nights as well. The extra stiff material around the zipper area to protect you from tearing the bag is also a great design by marmot. Lastly the extremely light weight and small stuff size is perfect for light weight backpacking/camping.
I would without a doubt recommend this bag to anyone looking for the best 40 degree summer/spring/early fall bag out there. I am currently saving to buy the Marmot Lithium for my winter bag since I am so satisfied with the Atom. (Might buy a Helium later on too)

After using countless crummy rental/borrowed synthetic bags I finally bought myself the Marmot Atom bag. While the price is a bit steep I absolutely do not regret buying it one bit. I have used it in temps down to the mid-30's and it has been perfect. It is incredibly soft and comfortable. Another benefit is that it has an extremely simple design: one zipper and one pull-cord. No jumble of zippers and cords and velcro attacking your face as you sleep like with so many other sleeping bags. The full-zip design is great for warmer weather camping as well as the dual-sided zipper for venting your feet if they get too hot. The hood is great for those colder nights as well. The extra stiff material around the zipper area to protect you from tearing the bag is also a great design by marmot. Lastly the extremely light weight and small stuff size is perfect for light weight backpacking/camping.
I would without a doubt recommend this bag to anyone looking for the best 40 degree summer/spring/early fall bag out there. I am currently saving to buy the Marmot Lithium for my winter bag since I am so satisfied with the Atom. (Might buy a Helium later on too)

I love its size and weight. I bought the long and it actually measured 1lb 10z, 2 oz more than Marmot's website states. That's still pretty darn light and compacts down to the size of two Nalgene bottles without a compression sack. I'm 6'1" 185# and the long is the first bag I've had that actually fits well plus I have room to wiggle. A bit pricey, but for the weight and quality of this bag, you can hardly beat it.

I purchased this bag site unseen. I was pleased with the construction and the full length side zip is great and the interior material is super comfy the bag just feels cozy. I am a warm sleeper and used the bag in a tent when the temps hit 32 being inside the tent I was just fine. The only real problem area for me on this bag was the fact I am to large for it not so much length wise but I really had to sleep mummy style in this bag which to me was somewhat uncomfortable.

If you are of average size with regard to your torso I highly recommend this bag.

This bag is ideal for summer use. It packs super small and is insanely light.

The 40 degree rating is accurate, I have used it right at that temp and have been fine.

Bottom line is that its worth every penny.