I think they're very convenient and I like them, but sometimes nothing beats the old tried and true physical book.
I have an enormous amount of PDF's on my computer now, most of them are anatomy, textbooks, and artbooks. I also admit that I've never paid for a single one. Granted the textbooks were free to download by the publisher, the rest not so much.
If PDF's do in fact count as ebooks, then I read them mostly on my laptop, as I don't have the money for a kindle or ipad, though I'd love to get an ipad, just not for ebooks.
I don't have too much of an issue reading on a screen, until a glare ruins it for me. Sometimes staring at a screen for long periods of time is tedious and annoying, and sometimes I wind up not reading as much as I had hoped. So when comes to reading for long periods of time I prefer the old fashioned book, and I also enjoy reading books more than on one of those eReading devices because of how it feels, like the feeling of paper between my finger tips is very comforting.