I'd first come across it on a recommendation from one of my aunts, who thought I'd enjoy it. She was familiar with the radio series and told me that the BBC had turned it into a TV miniseries, which was at the time being shown on PBS. I watched it and enjoyed it very much. (I later came across the original radio show she'd mentioned, on NPR -- something that I rarely listen to, even though I know people who broadcast various shows on it here in New York. I recognized the voices as belonging to some of the same actors that I was familiar with from the BBC TV series, which was nice.)
Later I read all the books, eventually acquiring the omnibus edition containing all the novels and bonus short story. I enjoyed Adams' work immensely and hoped he'd write more Hitchhiker's Guide books and was saddened to hear of his sudden passing in 2001. I even bought a copy of his "Starship Titanic" videogame and played it quite a bit for a while too.
I thought the movie was a pretty decent reflection of the prior forms of the Hitchhiker's Guide, although I personally liked the TV series Arthur Dent MUCH more than the current movie's. The actor who played Arthur before was Simon Jones, who does the befuddled Briton to perfection, IMO -- see him as Bridey in "Brideshead Revisited" for another example of him as a dotty lord who Just Doesn't Get It. I thought it was a nice touch that he did a cameo in the movie as the video "answering machine" of the planet Magrathea. I also enjoyed certain visual elements, such as the style of the Guide's animated entries, which I found to be pleasantly retro and reminiscent of the work of the artist Shag, a personal favorite of mine. Alan Rickman as Marvin the Paranoid Android stole the show though, far out-doing the prior BBC-TV incarnation by a longshot.
All in all, a very pleasant experience -- with luck, there will be more movies, such as "The Restaurant at the End of the Universe", to look forward to in the future!