I only saw the first few movies and a couple of episodes but, I have to say, Digimon has a really compelling concept. While the idea of parallel worlds isn't really groundbreaking, most children's series involve a separate world entirely, the real world exclusively, or hidden societies -- Pokemon or Angelic Layer, Cardcaptor Sakura, and Harry Potter, respectively, as examples from my memory. Digimon creates an entirely separate world within a comfortable universe, making suspension of disbelief natural and satisfying. The only other thing I can remember doing something similar is .hack, which has the weird discomfort of being nominally fantasy. Moreover, it lets the show handles "exploration" a lot more naturally, comprehensively, and believably than Pokemon or Harry Potter ever could -- comprehensive literature of the parallel world exists in both cases, whereas the Digidestined have only what they learn...
I'm not sure why I'm typing this. I really wish I had spent more time watching Digimon, though. The movies were really solid and I rather feel like going back and watching the entirety of the first few seasons.