I have read all five books of this increasingly inaccurately named trilogy many times over, and have been following the movie production for the last year. If it were not for the fact that I had to run my wife to Urgent Care last night, I would have seen this movie on opening day.
As a fan of the series, I liked the movie. However, I can easily see where you would not comprehend much of the humor from the movie of you had not read the books. That is the only complaint I have about this picture. It stays relatively true to the storyline of the book.
I was a bit surprised about the casting, but, of course, I had my own picture of each character already predetermined from when I first read the book. Tricia McMillan (Trillian) has a very subtle beauty about her character. Ford Prefect was much less arrogant than the book presented him, and Arthur Dent was exactly what I had pictured.
I would recommend seeing this in the theaters if you have read the book. And try and go with others who have read the book. (It was obvious that many in attendance when I watched the movie were already fans.) If you have not read the book, I recommend that you go do so now, and rent it when it comes out on DVD.