Yeah, I've read a bit
Around 70,000 pages at this point. Started with eReader. As better options presented themselves, I went on to others. Tried Stanza (put in a lot of time converting materials I had, etc) and abandoned it as too slow. I have used the Kindle and B&N readers, and they're fine. Kindle's probably the best purchase site in my opinion -- but user content is not well supported unless you own a Kindle.
The option I use presently is GoodReader. It has a wide variety of formats available, but its real strength is PDF. I use it to carry all my user content, and anything I can get into PDF. It handles huge files with no problems, and one can move content onto the iPhone/iPod touch with ease via the USB sync cable or (piecemeal) by the Web.
After being able to format things however you want in PDF, all the other readers are cheap-feeling and inflexible by comparison. After being able to copy whole folder hierarchies to the iPod at once, the one-by-one model associated with all the other players I have used is painful.
There have been a coupe of times that eReader got itself into an inconsistent state, unusable, and I had to delete it. All of my books went away as well, and I had to get all of them (hundreds) back, one by one, in a very slow process. GoodReader -- 1 drag and drop, 5 minutes.