I really want to like this series. My 6 year old daughter loves the stories, and they offer wonderful opportunities to explore various periods in history. Jack and Annie have adventures, solve problems, meet historical figures, and use their imaginations. What's not to like?
Well, it seems nitpicky, but I have a huge problem with the editing of the early books in this series. (The later ones are better, but we just re-read Mummies in the Morning and it's all coming back to me!) The fragmented sentences are out of control. There are so many that I stopped counting! I'm sure that it won't matter to young readers, but I can't understand why we can't expose them to grammatically correct English while they're learning to read.
I also agree with other reviews that fault the thin character development and level of factual content. I know that these books are for beginning readers, but I think there are missed opportunities to add real knowledge. Again, I think the later books in the series are much better, and the research guides are a great companion to the stories.
Overall, I would say that reading the Magic Tree House series has been a worthwhile investment of our time. It just bothers me from time to time when I think about how much better it could have been done.