I came to Air having watched two of Key's other anime series, Angel Beats! and Clannad, both of which I had enjoyed quite a lot with some reservations. Air doesn't live up to these two later series, and though it never quite indulges in the sometimes irksome mood whiplash of those two series (which at its worst can undermine the emotion on show), it still has a whole host of problems some of which Key would become more adept at reigning in with later productions.
The story of Air primarily concerns the character Mizusu Kamio, a girl who is obsessed with dinosaurs (frequently spouting the phrase "Gao") and who lives a lonely life for reasons we discover later on. She encounters and becomes friends with a travelling puppeteer called Yukito, and the main focus of the series is on her interaction with Yukito and also with her foster mother Haruko.
The problem though is the other storylines... As well as Mizusu there are also a few other girls who Yukito encounters. When they're introduced it becomes painfully obvious that Air was adapted from a H-game (which, admittedly, is a quality that thankfully entirely disappears in the shows second half) as each girl is clumsily introduced and set up as a potential love interest.
The problems caused by having these other girls in the show are multiple. Each girl gets the Key-style treatment of some sort of weepy catharsis as a issue relating to their past is discovered and is ultimately resolved somehow. Except, it isn't really weepy at all. We get so little time with each of these girls that I never really cared all that strongly for them, and the series is so desperate to cram these stories in so that it can get to all the stuff it has planned in its second half that as a result the stories end up feeling rushed and awkward. It's not that the stories for these side-characters are bad - actually they're rather good - but the presentation for all the aforementioned reasons stopped any opportunity of them affecting me emotionally. And then, when the series moves on to its second half, it almost entirely abandons these characters with the exception of utterly inconsequential and mundane cameo appearances.
The second half of the series does two things. The first is that it seeks to parallel the story of Mizusu with a story about a girl that occcured 1000 summers ago: a winged girl named Kanna. The thing is, I like the things the series is trying to do with this, but the execution was once again at best clumsy. For a start cramming these couple episodes in to the middle of the series like this hurts whatever little flow the story still possessed by this point: I'd have far preferred it if the information about the past was slowly revealed to us over the course of the series, in a similar way to how Clannad slowly informs us as the series progresses of the story about the girl and her mechanical friend. Another problem is that the tone of these scenes 1000 years ago feels somewhat different to the tone of previous episodes in part due to a few fight scenes which don't really feel right as part of the series.
And then we go back to the present day, though I won't spoil what happens (but will say that, despite my feelings about the series as a whole, Episode 11 is wonderful). The end of the series is definitely my favourite bit, but I have to say that I find Air immensely frustrating in its own way. Perhaps the most frustrating thing is how good it could have been. There are plenty of interesting concepts and ideas in it and if Key were to have disposed of the side-characters and instead spent more time fleshing the central story out - and doing so in a more fluid manner - then Air would probably have been a very moving, engaging story. Instead I found myself almost dreading watching the next episode not because the series is bad (it isn't) but instead because it isn't so far from being very good.