It's AWESOME. I don't think I would have liked it as much if I hadn't seen the show because when I read it, I was able to hear Michael C. Hall's voice during the internal dialogue parts.
The book follows Season One. There are some parts that are exactly like the series -- dialogue and everything -- but there are some differences, too, which kind of makes things interesting. It's the basic same story, though, about the ice truck killer.
Darkly Dreaming Dexter by Jeff Lindsay.
So good.