I didn't like this book right from the start, as I found the rather episodic nature of it a little bit difficult to deal with. Plus, Tom didn't come across as a very likeable main character at all - especially at the start of the book he seems rather mean-spirited and pugnacious. What kept me reading though were Mark Twain's very funny and apt observations about human behaviour - like his comments about the village people's behaviour in church, and his analysis of why people dislike doing chores (the famous episode of whitewashing the fence ;)). Around chapter seven though the plot finally picked up and from that point onwards I enjoyed the story a lot - how could one not, with a murderer on the loose, the boys going treasure hunting and playing at being Robin Hood and (eventually) getting lost in caves. Definitely a recommendable read, I think, for everyone who loves adventure stories.
And since all the critics seem to agree that Huckleberry Finn is even better than Tom Sawyer, I'm already dying to delve into the sequel.