It depends.
Leisure reading, I actually do enjoy the screen. I find my Kobo is easier on the eyes, and FAR lighter - I only hold it with two fingers. That said, I don't bring it with me, so if I want to read a quick chapter of my book, I'll just break out the iPad and thumb through it. It's good for a novel I want to barrel through.
The Kobo app is great in that it syncs my account between my Blackberry, iPad and ereader, so it keeps me on the same chapter all the time, with one copy of the ebook!
If I'm doing work for school, or instructional work I prefer paper. For studies it's good to be able to write in the margins, underline things, and take notes. I've tried annotating PDFs and it's just not good enough, for my money. For learning a craft or reading a non-fiction book on, say, procrastination (Piers Steel's "Procrastination Equation") I find e-books unsatisfying. You can't do the little self help quizzes, the charts and tables are clumsy. If I want to look at a footnote or end note, there's no easy way to flip from page 30 to page 602, and then immediately back to 30.
Also, depending on the book, many publishers simply have a hard copy scanned straight into the computer, and have it rendered as an epub file. The result is occasional hiccups where the machine guessed the wrong word from the font, and you have these sydlexdic hiccups while reading along. I've seen it everything from self help books, to Star Trek fiction, to Sherlock Holmes. It's a pretty consistent gripe of mine!
And yes, sometimes I enjoy the musty smell and the tactile sensation of old books.
