Thoughts, in no particular order:
1) This is talking about the six-episode BBC series from 1981, not the recently-released movie. Don't have much interest in the recently-released movie, probably will never bother to see it. It's not exactly to the book what 'I, Robot' was to Asimov, but I was a bit underwhelmed by the bits I saw nonetheless.
2) Sandra Dickinson is probably the single biggest factor in dragging this series down...but it's not really her fault. She's been saddled with an outfit that makes her look like one of Ming the Merciless' concubines, her hair is just relentlessly 80s in a painful way, and the director has told her to deliver all her lines in an American accent that a) she can't do, b) doesn't make sense for a character from Islington and c) makes her sound like Vanilla Whore in 'Scott of the Sahara'. All that together, and her ability to be convincing as an intelligent, centered astrophysicist dies upon hearing her first line (and it doesn't help that her first line is one that you'd expect to be delivered by an air hostess.)
3) The other big factor is, of course, The Other Head. Yeesh. The actor playing Zaphod does the same awful cod-American accent as Trillian, but it works better on him than it does on her, because he's meant to be larger than life, silly, and not too very bright.
4) The actor who plays Ford Prefect would make a good Doctor, if you went into a parallel universe and took the part away from Colin Baker. (Not that I dislike Colin Baker in the part.) He does have a tendency to underplay every line, delivering them all with this sort of weirdly soothing drone, but he gets the sensibility of Ford right, and that's what counts.
5) The actor who plays Arthur seems a bit too arch for my tastes (it's as though I want to take him and Ford and hook them up to a giant "Charisma Transference Device" and just take a little off the top for him and give it to Ford) but does do what he needs to do for story purposes.
6) Marvin is, of course, magnificent.
7) Production values are low, but then again, it is early 80s BBC. And they get lots of positive style points for keeping to the story's digressive, excursive, ramblingly brilliant style. The general myth about 'Hitch-Hiker's' is that it's horribly plotted, but very funny; the actual truth is that it's cleverly plotted, but that the plot is mostly going on while the characters aren't looking and they don't necessarily notice the plot even after it's happened.
8) It really bothered me at the time that they went straight from the Magrethea stuff to the Milliways stuff, instead of sticking to the source material. You may all now laugh at my youthful ignorance.
9) The actor playing Number One (of the Golgafrinchians) didn't understand he was in a part that was, by its very nature, impossible to overact in. He's way too subdued. (Although, again, this could be the director.)
10) I really, really miss Douglas Adams.