I finally got around to reading the book, How to Train Your Dragon, by Cressida Cowell, over the weekend as well. I had read, somewhere, that it was very different from the movie, and it certainly was. In fact, the resemblance didn't go much beyond the names of most of the main characters.
David Brin, in his blog, recently wrote that the main difference between fantasy and sci-fi is that fantasy generally goes back to the status quo, while sci-fi generally learns from the past, and moves on. By that metric, the book is fantasy, while the movie is sci-fi.
To me, the book is ok, but the movie is excellent. And having read many books on which movies were later based, this is a very rare experience for me. I'm told the movie for Fight Club is much better than the book, but since I haven't read the book, it's impossible for me to say.
The movie, I guess, is much more positive in its outlook, and Hiccup managed to accomplish a great deal more. It's a tad oversimplified in trying to train a wild animal (particularly in how long it takes, especially the non-Toothless dragons), but just fabulously well-done. The scenes with just Toothless and Hiccup are, across the board, flippin' awesome.
The book? Eh, it wasn't bad, but just didn't do much for me. I guess we'll see how my kids like it, when they're old enough to read it. But if you've only got time for one, I'd definitely go with the movie.