I heard about this about the time it came out, and got hooked a few weeks before Season Two began its US run. Though, as all cartoons do, it began a bit confusing for me (The first time I saw it (about the middle of the first ep) I thought Sissi was Yumi...), I caught on quickly.
Code: Lyoko is unique among cartoons. It is shown in both an anime style for the real world (which results in many-including some well-known sites- mistaking it for a Japanese cartoon, it is not, it is French) and a computer style for the Lyoko world. It has probably the longest distance between two seasons recordable (a year and four months for the US). And, it has-almost-no backstory. They don't tell us how Lyoko came to be, how Ulrich and co. found it, and why the computer has a virus. This is left to the fans-and Season Two, as we hope...
The show has realistic characters, an amazing plot, and a good eye for suspense. It does, however, have flaws over all this greatness.
First, bad dubbing. Like most inter-language imports, the dialogue does not match the mouth movements of the characters. The dialogue is often rushed, and flawed in some cases. One drastic example is Jeremie. Near the beginning of the series run, his voice was very high, but around the fourth or sixth episode, it changes, and it now sounds like he's trying very hard to sound older. (Puberty? Decision of the dubbers? Or perhaps he's just trying to impress Aelita, eh? Well, keep trying, man. The new voice sounds horrible.)
Second, scene reuse. This is abused heavily in season one. It's like the made scenes of everyone going into the sewers beforehand, and just used them where they were needed. This, for the most part, is discarded in Season Two, which is good. Although they STILL use that shot of Odd getting hit in the chest...
But don't get me wrong. Even with its flaws, this is a GREAT show. Not an EXCELLENT show, but a GREAT show. If you see it on, watch it.
But very carefully...