I've been looking at hydration bladders after a friend of mine used one on a 25-mile hike over lots of hills and it worked very well, while I ended up with one of my cycling water bottles in my hand as it turned out to be too big to comfortably put in a pocket and if I put it in my backpack I couldn't take a drink without stopping to faff with the pack.
Anyway the hydration bladder seemed like a great way to carry 3 litres of water, which would be a definite help on longer rides. It would also mean I could sip at any time rather than taking sips from a water bottle on the move, which I'm not overly comfortable doing when there's much traffic about. As a bonus 3 litres is equivalent to four water bottles and I've got two bottle cages, so at a push I could carry the equivalent of six water bottles.
The only thing is I don't like riding with a backpack on, so I'd really want some way of mounting it to the bike rather than to me. In theory I suppose I could strap it to the top of my pannier rack but I'm wondering if anyone else has done anything like this and, if so, how they did it and how it worked out.