Great for kids six to ten, but can be a little violent.
My six year old son "hated" reading, even when read to. He was feeling frustrated that he couldn't do it (he's only six), but he wanted to read the chapter books his older brother did. Magic Tree House and other series didn't engage him. While on vacation in Virginia, my wife found the first of the series at a local Barnes & Noble. There was a 3 for 2 sale, and my son liked the cover (both sons talk about the details of each cover).
Now he can't stop reading them (actually, having them read to him). He gets upset when it's time for bed. And, he's become really interesting in reading (he has a great first grade teacher, too).
What's interesting is that we've lent the books to two other "reluctant readers" we know and both can't get enough. One is in third grade and the "easy" books were too dull. My fifth grade son, who enjoys Percy Jackson and is an excellent bookworm, had to plow through them all as soon as we get new ones.
Although formulaic, it is not quite as obvious as Magic Tree House (our older son is a HUGE Jack and Annie fan, and we love those books, too). Each monster is different, with a different danger and a different solution.
Beast Quest also has a simple-yet-decent mythology. Both of my sons watched Beyblades for a while and spent a year breathing the Star Wars universe. There are plenty of characters and lore and rules that make the universe work. Beast Quest satisfies their need for a complete mythology that grows along with the series.
There is a lot of action and plenty of danger. Some reviewers have mentioned the violence in the books, and I don't disagree. The first series is mainly action, but people die in the later series. In the first series, the monsters are good but controlled by an evil collar (he unlocks each collar in a unique way). In later series, the monsters are just evil and have to die. Think about your own values and what your child watches for media. Star Wars and Clone Wars is as "adult" as we go, but that's pretty violent at times. But Harry Potter and Percy Jackson get pretty violent too (but are meant for slightly older kids). Be warned, as they are addictive.
The U.S. series runs out at #24. My sons had been anticipating at least 60, as they are mentioned on Wikipedia. Apparently, Scholastic stopped at #24 and so we have had to order the next 18 from the UK ($25 total for used copies, but another $75 for postage!). Come on Scholastic--publish these books!