For me, personally, the issue is shelf-space, in that I haven't got a lot of it. I fill them up with books then, inevitably, have to clear some out to make space for the new ones. I make regular trips to the local Cancer Research shop to donate the books I don't want anymore. Inevitably, down the line I decide I want to re-read something (at the moment it's China Mieville's Perdido Street Station) and find I don't have it anymore.
So I was given a Kindle for Christmas and have really been enjoying using it. It's allowed me to buy books I wasn't sure about, or try authors I hadn't read before, without using up that precious shelf-space, and I'm still buying treebooks by the authors I like to collect, or where ebooks aren't available. I'd say, since getting the Kindle, my reading has been about a 50/50 split between ebooks and treebooks.
Edit: The one big negative with Kindle versions, imo, is maps - they're virtually unusable and - liking fantasy as I do - I do want to look at them.