After getting the iPad I immediately bought a few ebooks and have yet to complete one. Hell, I've yet to finish a sampler. Between the eye strain and the draw to check RSS feeds, etc. this is the one area the iPad hasn't been a complete success for me. Well, the other areas successes might be the problem.
Although I wish the Kindle had native ePub support like the Nook, converting non-DRM files seems simple enough (and DRM ones I could care less about format). Every title I've checked has Kindle editions cheaper than B&N, iBooks, and Borders ebooks and all, including Amazons, paperbacks. I'm sure there are exceptions, but I haven't come across one in the last couple weeks (admittedly have only looked up a dozen titles).
Audiobooks to me is all about commutes. Increase or decrease that and their use adjusts accordingly. I listen to about a tenth I did five years ago. Well, that and podcasts have nudged them aside a bit - that damn This American Life app!
Tack on the weight difference with the iPad and I hope this is going to be a solution. Fingers crossed. if not, at least it will be a way to share ebooks with others. Toss them the Kindle/iPad while reading the other.