Maybe the problem with this is that some people are a bit too close to the original. This isn't a criticism, by the way. I just can't help feeling that when something is so perfect (which in the eyes of many, the original HHGGTG is), anything which tries to "reinvent" it isn't going to live up to expectations.
I don't know the radio series that well but enjoyed the TV series. I also enjoyed the film but I appreciate that it's different from the original incarnations. If you go to the film with an open mind, perhaps it's more enjoyable?
FWIW, I'm exactly the same with Doctor Who. When I heard there was to be a new series, I was both excited and dreading it because I didn't want it to spoil "my" series. When it was shown, there were the occasional places where I cringed a bit and thought, "Oh no - why did they do that?" but the positive aspects of it far outweighted the negative.
Anyway, I digress (to quote Ronnie Corbett!). So, funny films which aren't - I agree about American Pie (done before, and better, by Lemon Popsicle).