The major thing I was afraid of, going into this movie, was that the jokes would be too familiar. I have read the books and listened to the first two radio series often enough that I can quote entire chapters of the Guide verbatim. I own a "Don't Panic" towel. I know the jokes and I know them so well that they no longer amuse me, in any format. Yes, they are classic, but no longer amusing for their own sake, more amusing because I remember they used to be amusing. And when they did appear, most - not all - of the familiar old jokes did indeed fail to raise much more than a knowing smile.
But, and this is the key point here, the movie is, I would estimate, over 50% new material. And the new stuff was frequently hilarious. I'm not a baying fanboy who cries whenever something is changed. The Hitchhiker universe isn't designed for that anyway - faithfulness to the books is precisely what the trilogy is NOT about! The story has always changed significantly whenever it was moved to a new format, as well it should have, because the results of trying to stick too closely to the books (see the TV series and the new radio episodes) are often awkward. I welcome change. Zaphod with only one obvious head? Brilliant! The Heart Of Gold no longer running-shoe-shaped? Loved it! The revisions were mostly for the better. A significant number of Guide entries were ditched, which is to be expected because, let's be honest here, narration, while entirely acceptable in writing and radio, has very little place in film. I was surprised they didn't get rid of more, to be honest. Also good to see were a significant number of references which only hardcore fans would actually spot - I won't spoil them for you here in case you are a hardcore fan, which seems probable.
Stuff they got right
The Vogons and their planet. The casting - basically, every character was brilliantly cast, though Bill Nighy as Slartibartfast stands out in my mind. Deep Thought. The Infinite Improbability Drive and its effects. The ending! The Magrathean factory floor - appropriately mindboggling. Earth's destruction. Several of the older, traditional Hitchhiker jokes. Almost all of the new ones.
Stuff they got wrong
The plot. The plot was insane. I'm a hardcore Hitchhiker fan and I was completely lost at the halfway mark. Perhaps I wasn't paying enough attention, but it seemed to lurch from one situation to another without a huge amount of explanation. The pacing. No establishing shots of the various places they end up, oh, they're on this planet, whoosh, they're off again, why? No idea. Humma Kavula: served absolutely no discernable purpose.
It is an enjoyable movie. I would recommend you go and see it.
I think the main failing of the movie was its unwillingness not to diverge still further from the original books/radio shows. There was a whole lot of new content which Douglas Adams had come up with before he died, and they wanted to include all of it; but they also wanted to include as much as possible from the first book, and as a consequence they ended up trying to do too much in the time allowed. It got crowded and rushed. They just needed to relax a whole lot more. What was there was good, but could have used, say, another hour. Or the removal of Humma Kavula. I have nothing against the character personally, but he seems to be at the centre of a knot of untied plot threads and pacing woe.
I have every expectation of (and the greatest confidence in) a sequel.