I tend to watch all the extra features once, and usually, once only. The especially funny stuff, such as bloopers, might be watched each time I watch the movie. There's an Easter egg on the first Charlie's Angels movie that features Sam Rockwell doing a flawless impersonation of Christopher Walker -- very funny stuff, I always watch that one. There's also a great mini-documentary on the DVD of the Deepa Mehta film Fire, in which she talks about the riots that occured in India when the film was released there. I always watch that one too. I rarely watch feature-length commentaries -- if I'm going to spend a couple of hours with a movie, I'd rather watch it than listen to the commentary. The exceptions to that, of course, are commentaries to movies by Kevin Smith -- almost as funny as the movie itself.
When I'm buying DVDs, I make sure to buy the edition of the film that has (a) the best audio/video, and then (b) the most extras, unless it's something like The Terminal, in which the only difference is the inclusion of the CD soundtrack. Then I really don't care.
My favourite feature: the blooper reel, without a doubt.