For road biking I have a low profile camelbak, the Fairfax, which also has a sternum strap. It fits very well, doesn't move around, holds the same amount as two 700ml bottles and can also carry a few other small items like a couple powerbars, cell phone, patch kit, co2, etc. It sucks that my back gets more sweaty when it's hot than it would without it, but it's SO much easier to drink from and I HATE seat packs or carrying lots of heavy stuff in jersey pockets. I also hate that every time I want to take a sip from a bottle, I lose my line and possibly my momentum, plus bottles tend to leak and get gatoraid on my face, thighs, frame and bars. Bottles suck. I only carry them on super long rides so I can refill my camelbak, or on super short rides when I don't feel like messing with filling and later possibly cleaning my camelbak.

For real mountain biking I wouldn't even consider not wearing my camelbak but for super short and/or easy rides I'll carry one bottle. I just hate any extra weight on my bike, especially a full bottle, when going on technical trails or fast, bumpy downhills. But I also don't like the higher center of gravity with a camelbak.

So they both have their pros and cons.