OMG, I LOVED this book series when I was younger! It was by far my absolute favorite.
I read book 1 back when it first came out when I was in 5th grade, and read every single one of them up until about book 25.
And yeah... they are still an excellent read. Not anything like a typical kids' book whatsoever. Sure, at its core it's a good-versus evil battle against an alien invasion, but the themes that it touches on are VERY deep often, and I still absolutely love the characters. And K.A. Appelgate does a FANTASTIC job really describing what it would be like to actually be all of the different animals that the kids change into. Excellent sensory details.
And in case anyone is interested, the basic plot of the story is that there's an alien invasion going on. A race called the Yeerks, which take over people's minds by slithering into their brains and taking over their minds. They're trying to take over the human race. And as a last ditch effort to save humanity, a dying creature called an Andalite gives 5 kids the power to morph... to become any animal that they can touch for up to 2 hours at a time. And it's up to them to keep the yeerks from taking over the world. (And again, KA does a FANTASTIC job of describing all of the sensory details about what it's like to become all of the different animals. That is probably what I liked about this series the most as a kid.)
And yeah, I'll admit, I did write fanfiction where I changed into a girl, based on the continuity of this series... go figure.