I was a little daunted by this book at first. The Hitchhiker's Guide is so well known and loved that I was a bit put off--I tend to avoid things that are very popular (that's why it took me 4 years to pick up a Harry Potter book and why it's likely I will NEVER read anything from the Twilight saga...well, the popularity and the fact that they sound fucking stupid...another discussion for another time.) Also, I enjoyed the movie a lot (I think I was one of the few people that did) and thought maybe that would ruin the book for me.
For those who don't know, The Hitchhiker's Guide to the Galaxy is the story of Arthur Dent, an ordinary British man who discovers not only is his house being bull-dozed to make way for an expressway, but the entire EARTH is being destroyed (for an interstellar expressway, of course.) He is, at the very last moment, saved by his friend Ford Prefect, who turns out to be from another planet--sent to Earth to do research for the Guide. Arthur and Ford have a series of adventures, meeting up Trillian (a lovely girl) and Zaphod Beeblebrox, president of the galaxy and idiot extraordinaire. They fly off in a stolen spaceship, trying to discover the secret to life itself.
The book is highly entertaining, in particular the "explanatory asides" were side-splitting. My problems with it are mostly the same problems I have with the Dark Tower series: The book can't really stand on its own. Nothing is resolved, there is no real ending--it is quite obviously part of a series, and until I get my hands on the next book, it's like reading half a book. There are also some points where the quirkiness borders on annoying, but Adams manages to reign it in just in time.
On the whole, I liked this a lot, and would recommend it. The copy I have was put out in conjunction with the movie, and has some very interesting interviews with the writer, producers, and actors in the film, giving a new perspective on the process of making the movie and how they each saw their individual roles. Quite interesting.