I think I'd have to vote against vests in general... I really like the large, multiuseness of a pack, and generally have many pockets and belt-implements anyways for small stuff.
As far as the sweating goes, when the weather warms up, and sweating becomes less of a survival issue, I will return to a backpack (Probably, i've yet to become emotionally attached to this hippack, and I am to my old pack... ). I sweat under it, and a lot sometimes. I used it cycling and hiking, and once carried 5 gallons of H2O for a bunch of stoopid city-girls on a 100+ day. Lemme tell you, It's not a stranger to sweat. It just became a whole new story when I stopped, took my skiis off, started my car, and noticed it was _cold_.
I am wary of a camelback for very cold weather, the old one I had would freeze up after one downhill ski-run, and I never skiied in very cold (sub-zero) weather. I have my doubts as to how effective they'd be at -20... Anyone with experience?
As far as freezing water bottles, the experience I had with mine, I had pre-heated water, and a partially insulated holster. The top of the last-used bottle froze (The spout where you get the water from) the rest was a bit chunky, but liquid. I really dislike the idea of salting my water, but I don't know how much it'd make a difference...
Stryver