It was a fun, fast-moving series, in my favorite magic-school setting. But both characters and relationships were pretty derivative (especially in Book 1), and the final resolution came too fast and easy (not that I'm a fan of epic battles, but here, there was no battle to speak of). In fact, I believe that was the problem of the individual books as well – they were too short, finished too fast, before the characters could really struggle to overcome any of the obstacles. In other words, the had it too easy (okay, not all of the time, a certain death excluded, but that's my overall impression).
Still, I liked the main character and his struggle against becoming a villain, I liked the funny moments -- and I would have liked the whole thing even more if those kids have shown at least some signs of growing up in the five years the series spanned. Unfortunately, I didn't see any such growth; at the end, they still sounded – and acted – pretty much in the same (mostly immature) way they did in the beginning, i.e. like middle-graders instead of teens.
Well, I guess actual middle-graders would enjoy the ride, especially if they still haven't read Harry Potter.