I have read the books a few times, my girlfriend hasn't. We both thought it was great, as did everyone else at the noon showing yesterday, from the sound of it.
I guess you could sum up my review as a big "ditto" to Steve Lacey's review of The Hitchhiker's Guide to the Galaxy. I don't have as much history as Steve, but his review of the actual movie is so close to my own that I can't really add much.
As for Eric Gunnerson's review, I disagree with absolutely everything he says except that the 2nd head was "annoying. Very annoying." In fact, every spot that he says was not funny was apparently funny to most of the people at the theater I was in yesterday, because they laughed at every single thing Eric says was "not funny" or "no laughs".
Should you go see it? I don't really consider it optional for myself, there are certain movies I have to see regardless of whether I think I'll like them or not. The Hitchhiker's Guide is one of them. Luckily, it didn't disapoint me in the slightest.