I went on a bit of a book-downloading spree recently, having just discovered manybooks.net. That site is amazing because not only does it organize books from both the Project Gutenberg and Creative Commons collections, it also provides reviews for some of them and lets you download in a ton of e-reader formats (including Amazon's propietary azw format). I've yet to actually buy a book from Amazon, but I've already loaded over thirty documents onto my Kindle including some pretty large files (Ulysses, anyone?) which I will probably never get around to reading.
It's pretty fantastic knowing that one device (and an eletrical outlet) could contain enough material to keep me entertained for several thousand hours, but the problem with this of course is that I don't actually have several thousand hours free to spend reading. It actually makes me nostalgic for those times I've had to spend hours sitting on a bus from Ottawa to Waterloo or taking a super-delayed flight somewhere: think of all that time wasted without a magic pocket library thing!
(Incidentally, if you're planning on getting an e-reader, I don't necessarily endorse Amazon. Their file support is basically limited to their propietary format and semi-crippled PDF support which means that they don't support formats that work with library DRM. On the other hand, the e-ink screen is great, so if you're comparing it to an iPad... well, I'm not sure why you would ever get an iPad. I'd look into the Asus, Sony, and Amazon readers first.)