In addition to pretty much everything everyone else has said . . .
I've liked having instant access to "out of the way" books. A few times I've run across a book online that sounded interesting and been reading it literally a couple minutes later, even if it's not a mainstream book. In hard copy, at the very least getting a book involves a trip to a book store, but with books that aren't mainstream, it often involves either a special order at the book store or an online order.
For that matter, we're getting to the point that a fair number of books are ebook-only, so even if you don't fully abandon printed books, having an e-reader gives you access to some stuff to which you wouldn't otherwise have access.
Finally, to expand on something some other people were talking about, there truly is a lot of free, legal content out there for e-readers. Even if you limit yourself to the public domain, there's a fair bit of money to be saved by getting free ebooks versus purchasing hard copies.
But you don't need to limit yourself to the public domain. The
Baen Free Library offers tons of non-public domain books for free, usually the first book or two of a series, with the intention of generating sales for later books. That sort of free sample program, while not common, isn't limited to Baen.
I've even run into an ebook or two released free via the Creative Commons license. Specifically and appropriately,
Lawrence Lessig's Free Culture is available free of charge in PDF.
The point is that you can pretty quickly recoup the $80 outlay for your e-reader with quality, professional content.