Both. I liked the story more for the themes (call me a nerd if you will), the character development and the evolution of the character's motives. I liked it as a kid because it was real to me - it didn't talk down to me, and at that age, they did things I probably would have done similarly.
That was after I read a couple/few books though. Originally, I have to give it to the covers.
The first book I read was #3. I was a depressed kid, and I wanted an escape. I remember always thinking to myself that if I could be trapped as a bird, if would I take the choice. I also really liked how he wasn't just Tobias in a different body, but rather a hawk that was combined with a person. Hell, sometimes I wish I could be a bird or a wolf or something and escape from my life for a couple months - I'm not naive enough to think I'd never miss being human, but an extended break with nothing but instincts to guide me sounds... I don't know how else to say it, but extremely peaceful and romantic.
Come to think of it, that was one of the major draws to Animorphs - how morphs affected the character's emotions and motives in a subtle way, even after they were in control. It was eroded later in the series as they became more and more used to morphing, but I still remember how Rachel was more confident as a cat or how happy Jake was as a dog, even after they had subdued the animal's instincts.
Still, it wouldn't have been Animorphs without the constant paranoia and fear of the Yeerks and how everyone might be a controller. How hard it was to trust anyone and the constant feeling of pressing loneliness. I have to say, it all worked together to give a unique story with unique concepts and themes that still cause me to think today.
Either way, I still look out windows the same way I did when I was originally reading the series - not as a way to zone out or to admire the trees, but with a strong desire to just fly out of them and escape.
