It's the middle one of the Kyoto/Key visual novel adaptation trilogy, between Air and Clannad. (The original game came out in 1999, though.) It's perhaps marginally less distinctive than the other two, having a less coherent dramatic shape due to its structural problems, but it's still an interesting, emotionally rich work of magical realism.
The show's problem is that it's based on a visual novel with five girls and five separate storylines. In the game, you can only follow one at a time. If you want to see another storyline, you have to start again. Here, though, they tell all the stories, one after another. Fortunately they've removed the sex scenes, turning Yuuichi from a serial shagger into a serial bleeding heart, but even so it's a bit awkward to have him bond deeply with all these tragic girls, one after another. Apart from anything else, it's echoing itself a lot. (I believe the technical term is "repeated themes".) There's a lot of illness and people in hospitals. Three characters are amnesiac, including Yuuichi himself. More than half the girls are magical in some way. This would have felt appropriate and right in the visual novel, since that's effectively five linked novels, but chained together here it's a bit odd.
It works, though. It's emotionally powerful. However Tomoko found the show unwatchable, because of all the anime-voiced high school girls who'd have seemed childish even at half their ages. (There's a reason why the worst two offenders are like that, but Tomoko doesn't care.) That said, though, Kanon also has a strong female fanbase.
It's also a harem anime that isn't. Technically it's harem. One boy, lots of girls. However Yuuichi never lays a finger on anyone (although Makoto pushes that hard) and it wouldn't change anything meaningful to make him female. It's serial tragedy. Only one girl's story doesn't have death and/or terminal illness. Besides, the show's subverting the formula it helped to inspire of "boy meets troubled girls and solves their problems as they fall in love with him". Yuuichi is returning to the area after seven years' absence and it's going to turn out that it's him who unwittingly messed up some of these girls' lives in the first place.
There's also no fanservice. You can often judge a show by its panty shots. This one has none, even when Mai's killing demons with a sword and doing action leaps in a miniskirt.
There's another anime adaptation of this, the shorter 2002 Toei one (14 episodes including an OVA), which as it happens I've already seen and enjoyed. I went back and watched a random Toei episode for comparison. Ouch. That was a mistake. I've enjoyed both shows, but putting them side-by-side makes the 2002 animation looks cheap and ugly in comparison, while its character designs are closer to the game's. In other words, they're creepy insect aliens. Personally I'd find that unwatchable if I still had the later show in my head. Storywise, KyoAni's longer show is more relaxed and slice-of-life, with lots of comedy and Yuuichi teasing people. To me, it feels more lived-in and natural. That's the one I prefer, personally. However its comedy makes the less mature girls look sillier and there's a case for saying that the 14-episode show has tighter plotting and more intensity.
I've even seen it claimed that Nayuki and Shiori's stories were done better in 2002. On the other hand, though, Mai's story suffered so much from Toei's compression that it didn't even make sense, while Toei's Yuuichi lost most of his personality. KyoAni's Yuuichi is certainly an unusual harem protagonist in that he actually earns the right to be the main character. He's not outshone by the girls. He's rude, snarky and funny. His relationships with them feel real and he's the driver of his own story.
Most of the girls don't just make temporary couples with Yuuichi, but are actually part of a two-girl story. There's Nayuki and Akiko (if you think mum counts as a girl), Shiori and Kaori (ouch), Makoto and Mishio (the loosest of these connections) and Mai and Sayuri (practically a couple). Shiori's story is theoretically the most hackneyed, but the presence of Kaori makes it brutal and gives it the strongest character work.
It's a powerful show, although this KyoAni version spends a lot of time at its default level of lighthearted banter. It has a happy ending, but that's not the same as pushing a reset button on all the tragedies. It's normally a light, funny show, but there's a lot of darkness lurking just beneath the surface (especially if you're watching for the second time and noticing all the foreshadowing). There's a reason for every case of amnesia. I was slightly afraid of Yuuichi's problem with remembering his promises.
And then there's all the magic.