Here's my first review for my YA and MG reading, so I decided I'd use it for one of my favorite MG fantasy series: the Fablehaven series by Brandon Mull. The series started off a bit slow for me, feeling a bit too childish in the first couple of books. Around halfway through book two, though, it had me convinced, and I loved the last three books.
What does Mull do well? There's a lot of action, and there's variety to it. His magic works. It's not incredibly systematic or anything, but he doesn't pull solutions out of nowhere. The characters are also fun, especially some of the side characters. As far as the main characters go, Kendra can be a bit too perfect at times, and Seth can get annoying with his stupidity, but they are kids. I found they were the perfect characters for the story.
The basic premise is that their grandparents own a wildlife preserve for all the fantasy creatures in the world. The series is, at first, about their adventures on this preserve. Eventually, they end up visiting other preserves, and it turns into a "save the world" plot. Along the way, the characters grow up and develop as people, and there are quite a few interesting twists.
In all, it's one of my favorites, especially later on in the series.