I've seen the movie and I liked it very much, even more than "Hero" or "Crouching tiger, hidden dragon" (and they were excellent!). The movie is colorful, the actors are good, the plot is magical and captivating, lots of battle scenes and martial arts.
The one thing that you should keep in mind when you decide to go and watch it, however, is that there's one important difference between "Promise" and "Tiger/Dragon", for example. While the latter was mainly about the martial arts, magical swords and true warriors searching for their destiny and characters' relationship remained peacefully in the background, "the Promise", despite its many combat scenes and stuff like this, is a love story, a fairy tale. Which makes it sort of similar to the "House of flying daggers".
The flaws? Well, I suppose there're some, because nothing is perfect. But, personally, I didn't notice any.
It's a beautiful love story and it's worth watching. There's a right time for everything and there are not many bad movies....
Enjoy it!