You don't need an amateur literary review of this established classic, but here's a thought. I've been reading all of the volumes I'm putting in the kids library. As you might imagine, some have aged better than others, ("Last of the Mohicans" qualifies as a real chore). "Tom Sawyer" holds up remarkably well. There is great depth here, but also great early reader entertainment value.
A younger reader can just enjoy the adventure, excitement, and hi-jinks of Tom and friends. The stories are accessible and spritely. Older readers can get more out of the book, and adult readers can get even more. So, while it may be low-tech and a little dusty around the edges, this is certainly book that still has a place on the shelf.