I had the behind the seat bottles on my previous bike. Like Kimmyt, I also used them to hold the bottles for back up and not as a place to drink from. I did drink out of them while riding on occasion, and it was harder than drinking from the frame bottle.

I had the Minoura holder and my tiny wedge did fit underneath it, but it made it fairly inaccessable. I stored emergency stuff in it (tubes, gauge, etc)...nothing that I'd need regularly. For long rides, I used a small handlebar bag to store camera, lip stuff, multi-tool, snacks, etc.

You can kind of see it all in this photo (the rear bottles, the tiny wedge, the handlebar bag...):