I had no idea what to expect going to see this one and it really knocked my socks off. It's one big fairy tale that doesn't take itself overly seriously and yet it's intelligent, quirky and fun, and also a little dark and twisted at times. It's full of wit and charm, and the cast really makes it work. (Robert De Niro and Michelle Pfeifer really own the screen when they're on it and the princes of Stormhold are fantastic.) It has some dark undercurrents and a bit of grit to it, but that only makes it better. The original version of most fairy tales is pretty dark and not for children, and that's what this feels more like... the original, darker version of a classic fairy tale.
I liken it to the Princess Bride not because the story is really similar but because it has a similar sense of humour, especially towards itself, and has the same sort of fantasy feel and staying power. It's not the kind of movie I watch and then forget all about.
It's definitely a fairy tale written for adults and I've watched it a couple of times now without it losing any of its charm. In fact, I liked it so much I went out and bought the original novel and am in the process of reading that now.
If you like this one, you'd probably also really enjoy "The 10th Kingdom" which has the same gritty adult fairy tale quality to it.