That was me as well, I had an old rocket ebook that I loved, until the battery died and it proved impossible to get them to send me the right kind to replace it. It definitely has a couple of books I never read on it, although I'd tended to sticking to DRM-free. I read a lot of Baen on it.
At the same time, I had a PDA that I'd read eReader books on and a few lit books. Format shifting is definitely an issue. But I have, at any given moment, switched between reading books on a PDA, a dedicated ebook reader and on my Treo. Every once in a while, it's even the same book
Also, with my Cybook currently being dead, I'm back to reading on my PDA. So there is definitely some appeal in redundancy.