The stretch system of the montbell is perfect for me because I am thin and most sleeping bags create a lot of wasted space (wasted thermal efficiency). The 40 degree rating is probably accurate if a little optimistic, you will need to be layered heavily at that temperature but it does work. Would not recommend use at anything lower. Is very resistant to moisture, light, small and a great bag at the price.

This a great lightweight synthetic bag, not sure about the temp rating as I don't know what the temp was when I used it for the first time. However, we were up on a ridge.(Mt. Pleasant, VA)..i think around 4,000 ft the forecast called for temps in the 50s in a nearby town...not positive though...and it was super windy! With my Ibex wool beanie and my clothes on wrapped in a tarp I was more than comfortable. I ended up having to take my beanie off at one point because I started sweating!

This is my 2nd Montbell bag. The super strech is super awesome. My only complaint is that there isn't a collar like my 15F down bag. I slept on a thermarest in a tent on a 40F night. I needed to throw on some thermals, but I'm generally a cold sleeper.

Love the great stretch system of the Montbells and the synthetic handles the dew and wet conditions better than down. I'm 5'9" and got the long after trying the regular which was too short to be comfortable. Temp rating is optimistic though as it is very good down to 50 degrees, and only adequate at 40 degree. At 42 degrees I had wool thermals and a Montbell themawrap jacket, wool beanie and the Montbell thermal sheet and when the wind came down the mountain it was just OK if totally zipped up and head covered well. I was sleeping in a lightweight tent, too, but all mesh sides that don't block the drafts well. I definitely wouldn't rate it as a 40 degree bag, but if the temps stay in the upper 40's it works fine.
If looking for a good all around bag, this is probably not it as temps in the mountains often will drop to the low 40's even in summer around here in the Cascades. Taking it in the early Spring or later Fall with temps around 30-35 at night would not be a good choice when you add up the weight of extra needed to stay warm. My Marmot Col 0 degree bag in down weighs the same as the #5 Burrow with the Thermal Wrap sheets. So it is not weight or even size savings to take the Montbell #5 in cooler temps.

Took this bag on a 5 night trip in Indiana in May. Slept under an MSR Tarp on a Big Agnes Insulated Air Core and was comfortable. When stretched out can be a little drafty if its also windy out. (Easily solved by sleeping in a tent.) I am 6'2" - 200# and slept most nights with my head bundled in the hood cause of wind and my feet didn't bottom out. Had zero problems with condensation.

I am a restless sleeper so i got it based on the fact that i am always moving around in my sleep and it does that well.

nice and very compact and light weight. easy to roll up and stuff in stuffsack. didnt keep me that warm in 40+degree weather though. still cold at 50degrees.