Thought I'd post my thoughts & experience with these...
Have had one such bag for some 10 years, but it's seen relatively little use. Anyway, the concept is sound, and I really like the waterproofness. (Yes, the bottom is w/proofed). But the fact that it is rubberized can make it a clammy package, because it can get sweaty if you're warm, or if the tent is warm. Mobility is great, none of that "constricted" feeling some tighter bags will induce.
At some 3 kilos it isn't *too heavy*, but there's no way to compress it properly, so it takes a lot of room. As far as the 3 season rating goes ... well, I can't recommend it for anything under +5. Have had teeth chattering nights at -6, and event at +2, and these in a tent...
Anyway, pros and cons (subjective) noted on exercises and in general use:
+ Mobility of the arms = very "handy" (use *warm* gloves, and you'll be fine)
+ Doubles as a warm parka (no need to carry a separate coat) eg. at the campfire, camp guard duty, stargazing etc. (too squeaky for OP)
+ Summertime you can skip using a roll mat
+ Impervious to showers (no need for a bivi bag)
- Packs slowly (packing it in the integral hood, a tight fit, takes time). If you're out camping, that's ok, if you're on exercises, you don't want to spend the time.
- Noisy, it squeaks. Nobody's expecting a soundless tent, but you get quite conscious of the squeaking.
- It is a bit short, if you're over 185 it might be a bit constrictive
- Cold (then again, if you don't plan on using it below freezing, that's no prob).
So all in all, it's a sound idea, and it works pretty well in warmer climates. If you're alright with it being a bit cumbersome, go ahead and buy one, or two. It is a versatile bag. That being said, I've moved on to lighter weight bags (eg. an Arktis Halo 3 + a bivi bag clocks it in at 2 kilos, so I save a kilo).