I use a CamelBak "Mule" that holds up to 3L of fluids and I find it very useful when road cycling. I find that I drink quite a lot (did 70 miles on Sunday and went through the whole 3 litres over the course of the day) and so the ability to carry a lot of drink in one location is good.
I also like the fact that I can stash about 4 or 5 spare inner tubes, lots of tools, energy bars / dextrose tablets, mobile phone and waterproof in the bag as well. This keeps my bike as unencumbered as possible.
Yes, they do make your back get a lot warmer than it would unencumbered, it's not uncommon for me to get a few heat lumps after wearing it for a few hours. However, it is not so uncomfortable as to prevent me continuing to use it.
Some of the replies here have said that it causes them back pain. I'd suggest that these people should adjust the straps so that it sits more comfortably. If you have weak core muscalture anyway, then this kind of back-pack isn't for you anway, as it will only exacerbate this weakness.
When cornering I found I needed to get used to the slightly higher centre of gravity wearing one of these things causes, and you must make sure that the chest straps are done tightly. The last thing you need it your back pack shifting unexpectedly when whizzing around a bend.
For touring I say that they are great. For racing, probably not.