Why not the free-as-in-beer adobe acrobat reader? Well, because it is slow, confusingly bloated with features, and I have never, ever liked the interface. I used to go as far as rendering files to graphics and viewing them in a web browser, rather than use Adobe acrobat. I mean, who would trust a bat with anything, and at that, an acrobatic one?
Anyhow, Sumatra is slim, and a little bit quirky, but I don't get that raging anger when I use it, that I do when I use just about anything else.
I tried CoolPDF, but it kept bugging me to convert a PDF to an EXE, and it seems sluggish.
Sumatra is snappy, and mostly unobtrusive (keyboard-wise scrolling could be better...).