I am in the exact opposite situation as the OP, but have the same questions.
I have been a long time library patron who maximized use of all the interlibrary loan option out there. WorldCat is a wonderful thing. However, last Christmas my MIL got me an iPod touch and since then I have acquired and used just about every ereader app out there. I also truly believe that my tax dollars are used to support the library, so I have had no problem with checking out ebooks from all 4 of the Overdrive libraries I have access to and reading them on my touch. Bluefire and OMC have changed the landscape for reading library books on iOS devices in just the last two weeks. They've just eliminated a couple of steps from the process.
However, my MIL was careless with her shopping list this year and I am pretty sure I'm getting a Nook for Christmas. That takes care of the library issue there. So I got my wife to try a Nook in the store the other day and she hated the touch screen. Figures. So it looks like she'll prefer the Kindle. She really wants the 3G option so she can surf the Net while we drive back and forth to the inlaws' house.
So, all of a sudden, it looks like I'm going to have more ereaders then I ever thought possible. But I'm comfortable with Calibre and I really don't have a worry about the format issues.
As it turns out, one of the brick and mortar libraries I belong to is having an ereader fair tonight. I'm gonna go play dumb and see how much they actually know about the subject. It could be fun.