For a series of any genre, especially in romance, slow-paced, or drama genres, to be considered "good," it is necessary for the series to either have something that sets it apart completely, or for it to hit all the major aspects expected in its respective genre and excel at them. Air definitely manages to do the first of these, and at least comes close to the latter, as well.
The series starts off somewhat slowly, and, story wise, averagely, following a vagabond puppeteer. After a few episodes, however, it begins to do original arcs between the male lead and female secondary characters. After several of these you are finally taken back to the beginning of the "1000 summers" that the story promised, and taken into a full fledged fantasy. This is where the series truly begins to set itself apart. Characters that are only around for one or two episodes are managed to be fleshed-out extremely well, and many secrets about the leads are revealed.
This being said, after this is finished, many unusual things take place, and, in my opinion, manage to get the viewer even closer to the characters. Finally, after it is all finished, it leaves the views emotionally attached to the plot and characters, and satisfied with the series.
Apart from the story, the series has acceptable voice actors, strong character development (although somewhat wasted on a few characters with very little to do with the story), a great sound track, and amazing artwork (apart from the slight google, wide set eyes Kyoto is so fond of for females). All-in-all, a definite see for anyone that enjoys Key series or dramas.