I've never played it on the Wii, but I own the PS2 version, and it's one of my favorite games of all time. It's very stylistic, obviously, and the gameplay is EXTREMELY Zelda-like. It's just as good as Zelda games, and it even outdoes them in some respects (I enjoyed Okami more than Twilight Princess). Some of the dungeons are a bit simple, but the boss fights are great, manipulating the environment with the paintbrush is a really unique and interesting mechanic, and it's a bit more story and character oriented than Zelda games. The only real downsides are that the middle of the game drags just the slightest bit, sometimes the paintbrush controls aren't quite as responsive as you'd like them to be, and like Triffy said there is one boss fight that you have to go through three times throughout the game (the exact same fight).
Overall, it's an absolutely incredible game (ignore Triffy, he has no taste in games =p) and it's a shame that the game really didn't get the sales and attention that it truly deserved. Add that to the fact that Clover Studios disbanded, and it's unlikely that we'll ever get a sequel.