It's about cyberspace, augmented reality and children. You might expect a 2007 tech angle to feel hilariously dated, but that's not the case at all. It still feels with it today. It predicted smart glasses and its brats are as good at hacking as anyone (except Granny).
It was an award-winning cult hit in Japan and was regarded as one of 2007's best shows, but it took forever to sell internationally. This might partly be due to the tone. The cute art and child heroes might make you expect a kiddie show, but it's quite an abstract, philosophical work that's more like Ghost in the Shell or Serial Experiments Lain. (Even the show's fans, though, tend to say that their favourite episodes are the more whimsical ones like the microscopic sentient beard AI bugs or the one where a boy's best friend is a bug in the form of a plesiosaur.)
Personally, though, I think it's because Isako has likeability issues.
She's important. The finale's built around her and Yasako. Unfortunately, though, she's also a cold, rude manipulator who uses people like tools and will reject anyone who tries to get close because she says she doesn't understand the idea of friendship. Her speech patterns in Japanese got on my nerves. We'll ultimately discover that she's got a missing brother she cares about and a possibly-evil mother and... naaah, sorry. Too little, too late. Don't care. So she's lost in cyberspace, is she? Good. Yasako wants to rescue her? If you must, Yasako, but don't expect me to be interested. (As far as emotional engagement for the audience goes, she's trumped by an AI simulated pet.)
The rest of the cast is thoroughly engaging, though. Seeing small children running around doing hacker things is far more entertaining than most hacker stories, as well as underlining and making a point out of the inherent immaturity of what they do. (Sometimes to the point of vandalism.) Yasako's the most feminine and mature of the children (and by "most" I mean "only one who's even remotely") and a thoroughly nice girl. Her little sister Kyoko loves pointing at things and saying "poo". They also have an ugly AI dog called Densuke that's stupid even by dog standards. Fumie is a bundle of trouble. Daichi obviously has a crush on Fumie, but his way of expressing this is to be rude to her and make her life hell. He's been doing this for years and she thinks he's a pain in the arse and her number one enemy. There are plenty more children, of course, and they're almost all a laugh. The only one who's not (except Isako) is Haraken, who's been in a state of industrious depression since a girl died. He has plans.
The visuals are deliberately drab and grey, but the children are charming. They're the reason you'll stick with this series, unless you drop it during the last half-dozen episodes or so.
The cyber-philosophising is quite good. I prefer it to Ghost in the Shell, for starters. It's doing an interesting combination of William Gibson and ancient Japanese folklore, e.g. those paper seals you can throw to foil security software. E-space turns out to share features with the afterlife and fairyland. Bugs can seem alive. The show's also aware of real-world issues, e.g. children violating privacy laws, or virtual pets crowding out real ones, for instance. "Ah, a real dog. You don't see many of those."
Also, the city's Internet Bureau has giant search-and-destroy programs called Searchies... but they can't enter schools, shrines or private houses, because of bureaucratic rivalry. Those are covered by a different government department. The children can hack this by drawing a shrine gate on the road in chalk.
It's a clever, thoughtful show, with all kinds of tensions between "dry and high-flown" and "idiot children having a laugh". Unfortunately, the driest bits are at the start and the end. The show only becomes fun around ep.4 or so, while unfortunately the end is a bit boring. Ultimately, the show's okay. I don't love it. I liked a fair amount of it and I basically agree with the critical consensus, but I'll never watch it again.