It's quite interesting, for being so massively different. It's almost a category error to compare this with the Kyoto Animation TV series that's theoretically based on the same story and appeared the same year. I quite enjoyed it. Toei really didn't like the magical elements of those Key visual novels, did they? It thus seems odd to me that they chose to adapt them, although in fairness for all I know the arch-rationalist was Osamu Dezaki. (He directed Toei's movies of both this and Clannad, although not their Kanon TV series.)
Stuff that's the same:
1. a drifter called Yukito comes to town with a puppet, the ability to make it dance for children and no other source of income. (He may or may not be controlling the puppet with telekinesis. The anime never gives us any other explanation, but it's backpedalled so hard away from all the other magical story elements that I imagine many viewers just assumed there was a trick we weren't being shown.)
2. a girl called Misuzu lives with her mother (?), Haruko.
3. a thousand years ago, stuff might have happened with a swordsman and a girl with wings. This gets a fair amount of screen time, but it's just a story in a book as far as our heroes are concerned. Maybe it happened like that. Misuzu thinks it did. Yukito might too, but the audience are free to draw their own conclusions.
4. the music.
5. all the female voice actors are the same, while the male voice actors are different. Osamu Dezaki did that in Clannad too... ah, no, I'm wrong. The movie uses the game's voice actors, while it's KyoAni who recast the men. Ah well.
...and that's it. The Toei plot is similar to Key/KyoAni's, up to a point, but all the supporting cast are gone and we're in a prosaic world where nothing magical happens. (Even the visuals reflect this. KyoAni's town was a hallucinogenic dreamscape of sunsets, ocean views, rippling fields and other vistas representing air, flight and freedom. Toei's town though is usually pouring with rain, representing claustrophobia and being trapped.) You can choose to see the Key/KyoAni events still happening underneath this film's bald telling of events, if you really want. There's the crow. Misuzu's illness still has no cause that doctors can find. However the film's stripped a complicated, mind-bending magical fable down to its absolute bare bones of who lives and who dies.
Bizarrely, this makes the film quite simple and slow. It doesn't feel compressed, because it's discarded everything peripheral. There's no Hijiri, Kano, Minagi or Michiru. It's also less funny, albeit not completely po-faced. (Haruko made me laugh when she thought Yukito and Misuzu might be up to something in Misuzu's bedroom.)
This is clearly a less interesting version of the story, I think, but that doesn't make it bad. It works, I think. The artwork has dated better than that of the TV series, with everyone looking normal instead of being 80% eyeballs. There are no issues with 2005 digital line quality. The characterisation has some nice subtleties, e.g. Yukito's dislike of cowards. This Yukito is also a better children's entertainer than his TV series counterpart (despite his dislike of people and crowds) and you can almost believe that he manages to stay alive on it.
Sad fact: Misuzu's voice actress, Tomoko Kawakami, died in 2011 of ovarian cancer, aged only 41. She'd also played Utena in Revolutionary Girl Utena, Hikaru in Hikaru No Go and many more.
I definitely preferred it to Dezaki's Clannad movie, although they're very similar in approach. Personally I think the difference is that for me the Clannad movie is far weaker than the TV series, while expunging magic from it doesn't make that much difference except to end the story a bit earlier. Here, though, it's like night vs. day. It becomes a different genre, smaller and more focused. I found it very odd seeing a non-supernatural Air, although of course I realise that this opinion was formed by watching the TV series first. You could blow people's minds by showing the latter to someone who'd started on this film.
Personally I'd have assumed that this was a more accessible telling than the TV series, assuming you don't mind tear-jerker endings and a plot where nothing happens. It's sad, emotional and pure in its simplicity. The TV series is the one I'm more likely to rewatch, but I still respect this version.