I have the Fastpack. I ended up replacing the bladder with a 3 liter model and a better bite valve. As far as the mounting, I got tired of fitting it between the sports rack and the back of the seat. Now I just strap it to the bottom of the rack. I run the tube between the rack and seat, loop it behind the bag I usually carry, and clip it into the headrest support tube. This keep the length and angle conveniently placed over my left shoulder so I can drink easily.

Oh yea -- I put 2.1" MTB tires on it for the winter. It helps with icy patches! Kind of tricky getting those fat tires balanced in the fork, though. I'd say 2.0" is the max size for convenience sake.

Enjoy your new ride!