I have read all of the Hitchhiker books and I really like them. As for the TV series, the first episode was pretty good, the one where the Vogons destroy the earth and Arthur and Ford escape. But the series rapidly went downhill from the second episode onwards.

Hopefully the movie will be good, but there is no way they could fit the entire story into the length of a movie...
A lot of the stuff that happens in the books is just too weird to be in a movie, and people who haven't read the books won't understand, and it will just seem absurd and inexplicable. For example, like the missiles turning into a whale and the petunias thinking "oh, no, not again". Not that it's not strange in the book.

(Well, yes I know there is an explaination for the petunias much later on.)