I'm not sure what to call them, but they are one of my favorite bands... (along with many others, dream theater, opeth, in flames, etc.) They arent Prog and they arent quite metal. They have a hell of a lot more talent then nu-metal though thats for sure. Its very evident in the drumming. Danny Carrey does rank up there with Portnoy and the likes. The guitar is defintely not very complicated, but its far from the generality of nu-metal. The riffs are creative and very originial, although easily to replicate. The singing is some of my favorite singing in any band, I dont know how to classify it, maynard is just a great singer. The bass is about the same as guitar. I hate when they are categorized with stuff like deftones, becasue they are really a lot different. Kind of like radiohead in an alternate universe.
I agree with the post above me as far as categorizing goes.