Nathan reviewed this book awhile ago, read his review. Some of the jokes did feel a little stale by the end, but I still enjoyed reading them. Adams did a wonderful job weaving together some very perceptive observations with a well-characterized cast, making the 800 page journey an entertaining read. If you pick it up not expecting to gain any huge insight into anything, you'll probably enjoy it. Basically it's a good vacation read if you don't feel like completely throwing intellect out the window.
The last book doesn't really come to any closure, but the fourth book, So Long and Thanks for All the Fish, was definitely my personal favorite. It's relatively independent from the rest of the series, so if you can only read one, make it that one (you'll probably still miss half the jokes though).