I like PDF's, but there are limitations.
1) PDF's are cheap. That means when I buy book X, and it sucks, I've only wasted $5, not $12+shipping or whatever. I've also used it to preview some books, usually Malhavok.
2) PDFs, if formated right, can be great for printing out some things. Especially in a book with prestige classes, where they're broken up by pages (page count means nothing to a PDF) you can just print out the one or two you might actually use, instead of carrying around an entire book. (for example, Psychic Weapon Master was free, but still easily printed out and inserted into my Psi Handbook for reference.)
3) I hate reading stuff on the screen. This is especially irritating for the high art stuff. I was disappointed when the "text file" edition was deleted from the zip file. It was easier to print, easier to reference. But, then again, the art does make the books easier to read.
4) the lower barrier to entry. This means any guy that thinks he can write a sourcebook CAN write it. But sometimes that's a good thing! Some of these authors would never get through to a paper publisher. Most publishers don't even accept unsolicated stuff from someone not already known.
That said, PDF's are getting pretty expensive from some of the different companies. I'm not paying $10 or more for something electronic, especially when it'll be out in print soon enough.
Also, some of these companies have the worst descriptions I've ever seen. It's gotten a little irritating to buy something because it looks cool, then find that it's not what I thought.