This book is a great place to pick up the Discworld series, if you haven't read any of the others don't feel threatened by it being the...like 9th book. A whole new cast of characters is introduced along with the wonderful Ankh-Morpork. It's more than just a comedy about a dragon running through the streets. (With Pratchett nothing is that easy.) It has some serious commentary on Law and on human behavior. I see Pratchett as a kind of teacher, who hides his lessons in his hugely entertaining stories.
All that said, buy this book!! I apologize for all of you who will read it and need to read the rest of the series as soon as possible. It will cost you some money (he's got somewhere around 26 Discworld books alone) but you may end up thanking me.
The best character is Captain Vimes. When the book starts out, he is at the bottom of the pecking order of the city. He begins to straighten up, though, and near the end, he is a competent and witty officer of the law. There's a good part where Vimes has two men put in jail, and when he is told that the jail isn't very strong and that the prisoners will eventually be able to break out, he responds "I hope so, because the very first drain we come to, you're going to drop the key down it."
The other characters have their ins and outs. Vimes is the star of the show, but the rest of the Night Watch has its moments. Lord Vetinari, ruler of Ankh Morpork, is also a good character. His dark philosophy and strange opinions on how to run the city make the book even more fun to read. And the dragon just makes it better.
So overall, GET THIS BOOK, it's one of the best in the Discworld series, and should not be missed.