Loaded with tons of revolting gore (courtesy of some great special effects) and a few awesome Easter eggs (music cues from Predator, ftw), Slither is a fun, wild, and pretty entertaining horror comedy homage to the schlocky delights of the late 1970s/1980s from the early works of Cronenberg, Troma, to stuff like Night of the Creeps or The Thing.
I kind of would have prefered a more straightforward film that just happens to have some funny moments over a horror comedy (as it is a difficult thing to do well), but I had fun watching this. It's not perfect, but for what it is, it is good enough.
The cast is pretty good (it's nice to see Michael Rooker again), and Banks and Fillion look like they're having fun, so that's good. It was a nice little treat to see Jenna Fischer, but I wish she would have had a bit more screen time. Oh, and for the curious, this is supposed to be a campy film, and yes, the actors are acting that way on purpose.
Even as a genre exercise, this film is a little rough, but James Gunn shows some promise, so that's cool. Oh yeah, the squeamish probably shouldn't watch this; some of this is pretty gross. If you like gross though, or at least have a good sense of humor, then you'll probably find something to enjoy.