I have been using this pack for about a year now and personally I love the way these packs fit, they are very light and comfortable with tons of features. I sweat a lot in summer and although the design of this pack won't stop that (only cold weather can do that), it reduces it a lot! The only thing I can think of that I would like that this pack doesn't have is a daisy chain or more places to attach things to the back of the pack (and that is mainly because I use it for winter as well as 3 season).

I have three packs, a multi day pack (which I got before I found Osprey packs and can't justify replacing), the Atmos 50 and 35. Each has their own place, I use the 35 for most hikes and the 50 for light overnights, winter day hikes (not what the pack is for, and although it works it isn't what I would recommend) and hikes where I want to bring something that takes a lot of space (for example I once took ice cream to the top of a mountain and I needed the space to fit the cooler with dry ice as well as the usual gear). This is usually overkill for light day hikes and it isn't large enough for multi day trips unless you are a real minimalist.

You should go and try an Osprey pack at a local shop to make sure you like the feel, some people like the way that Gregory (or some other brand) packs fit better, but if you like the feel you will love this pack!

I use this bag for my daily commute, I love all of the pockets and small chambers and that it is waterproof. I love most things about this bag, however I tend to live out of my bag and the medium just isn't big enough for me (maybe because of the space that the laptop compartment takes up), I should have at least gone with the large. If you go straight from work to home you will be fine, but if you go from home to work to the gym to your girlfriends house then back to work and maybe make a stop for groceries or something, seriously think about the large bag.

I have used the Rho alpine and xc skiing as well as on many hiking trips with temps ranging from around 50 degrees (F) down to 0 degrees (at the base of the mountain). This layer is amazing in the range of temperatures that it covers, I rarely overheat and yet it seems to keep me warm when the temperature drops. It is very comfortable and dries quickly, I have an average build (5'11" around 175 lbs) and it is snug on me, but very comfortable (the way a base layer should be). I am planning on getting a couple more of these and making this my standard base, it is a little pricey, but well worth it.

I got two of these bags recently and tested them on a trip to VT with my girlfriend. When we set up camp it was 7 deg F, and that was around midnight, so I imagine it got a bit colder than that. She got a little cold and grabbed one of our 25+ deg bags and layered them while sleeping in a base layer and sweater (and then was fine aside from cold feet). I just had on boxers and boot liners and just this one bag, and aside from my feet being a little cooler than I would like in the AM (not uncomfortable though), I was nice and toasty except for my nose (go figure).
I read another person's review that said they couldn't sleep on their side. I am a side sleeper and had no problems at all at 6' tall and around 175 lbs so I don't know what the case was there.
I was a little thrown off by the lack of down on the bottom until I realized that down needs air to work anyway so having none underneath is fine (and it didn't just work in theory).
They are a little on the expensive side, but if you catch them on sale they are well worth the $.

If you are looking to not slip while shoveling the driveway or walking around town these are great. If you need traction on serious ice or snow while hiking or adventuring you would be much better off with microspikes, these just don't cut it. I never use them on the trail.