There is a _lot_ of truly terrible non-fiction for children out there. (Of course, there's a lot of bad non-fiction for adults too. . .) My biggest complaint with most histories--lack of a sense of humor. Fleischman doesn't have this problem.
This book, while definitely not a straight-forward biography of Twain, is going to be much more likely to get kids interested in biography and Twain. It's not fabulous, but is enjoyable and readable. Loved all the illustrations and breezy style. And for someone who's never read a full-length bio of Twain, it certainly left me wanting more.