Wow.... so while this might have its flaws, as an experience it's overwhelming (in a good way) and frequently stunning, as well as something that goes by pretty quickly for three hours.
Despite the epic length, it doesn't feel like a traditional epic. It more felt like two - or maybe even three - plots sort of smooshed together. And on top of that, each third of the film is very distinct. The first third has no action and is sort of like a period drama/mystery. The second third is more straightforward martial arts fare, feeling very similar to another of King Hu's films, Dragon Inn (which isn't necessarily a bad thing). The final third retains the action but takes thing in a really interesting surreal/spiritual/maybe even psychological direction. It's hard to put into words, but it leaves an impact.
I wouldn't call this poorly written, but the distinctive acts and more than one plot all combined together did create something of a disjointed feeling. It didn't make the movie harder to get through, as I only realised it wasn't entirely cohesive right at the end, but it's worth mentioning. The first third might have been a tad slow, too. But after an hour, there's no point where it feels like things drag.
Thankfully everything else is close to perfect. For starters, it's visually stunning. It has some of the best lighting I think I've ever seen, with night scenes looking realistically dark (without things being impossible to see), and all the scenes during the day are just gorgeous. All the great landscapes help, too.
The action satisfies and all the performances and characters are quite good. Very good music, too. On a rewatch I might not be bothered by some of those unusual screenplay decisions, but even with that minor criticism, this is still an amazing movie, and probably among the best martial arts movies I've ever seen.
