For what you read, what do you look for in programming books? What drives you to buy a physical book?
Tablets: Good for books with lots of visuals. I need to see the images, figures, colours etc... of the articles I read on the two magazine subscriptions I have. I sometimes forget the magazine at home when I want to read it (on the bus, waiting, etc...). Audio readings are a great feature on some magazines.
Kindle: I use it for reading most of my books now. Cheap, instant and very light to carry. The problem? The touch interaction is nowhere as convenient as a book. It's hard to navigate the book and I usually like to go back to previous sections. I avoid that all-together on the Kindle. I also bought a book on design (Design for Hackers by David Kadavy), and I wasn't able to see a single image. For the few that showed up, they weren't coloured which made no sense. Most of the books I use though have no images, and so navigation is the biggest issue.
Physical Books: I buy a physical book when I know I will only read at home. Books are almost always going to be the most convenient, easiest to use. Unless Kindle comes up with something better. The only issue is carrying the book around, having to store it (an issue if you move around a lot like me), and eBooks are cheaper. I also go to college in a town with not a single decent bookstore around me. They all sell college textbooks only. eBooks are instant while books typically take a week to deliver to me.
1. The fact that I can give it away to a friend.
2. The fact that books almost never contain advertising (the last holdout).
3. The fact that I am often out of reach of the Internet -- and even electricity -- for a large percentage of each year while traveling.
4. The fact that I am in my 60s and, as they say, "You can't teach a dead dog new tricks". :)