There were a lot of things I liked a lot about the movie; a few things made me laugh so hard the teenybopper in front of me gave me dirty looks. (Shut up, little girl, you were squealing at the mention of "Angel" in the Serenity trailer.) The Vogons were just about perfect, the iPod-esque Guide animations were very good and consistently funny, and the bit with the dolphins was a hoot. My only real problem with the film was that Zaphod and Ford seemed mostly out of character through the whole thing. I don't think it was a fault of the actors, they both seemed to be doing exactly what the script and director called for -- I just think the script and director didn't really understand Zaphod or Ford.
And yeah, I gotta say, the Zaphod heads thing ... didn't work. A CGI or matte-job second head on the shoulders would have worked much better. Heck, even the paper-maché Mark Wing Davies head from the BBC TV series would have been welcome.
So anyway, the final judgement from me is, taken on its own merits, it's an enjoyable film and I recommend it. As part of the larger body of Hitchhiker's Guide work, it's a relatively minor entry. Sort of like Fox's Doctor Who ... nice try boys, points for effort. :)