tl;dr version: 9/10 game which is greatly enhanced by the laser precision of the wii. Definitely buy this if you've never played it, and consider it anyway even if you have.
Okami is the best game I've played for the past year (previous year was Twilight Princess, no surprise there.) I didn't own it on the ps2 (only got to play at friends house) and I can hardly find fault with it. If you played the ps2 version of Okami then you probably don't need to buy this one, but if you have not, then this one is probably definitive. It's just simply fantastic, and it's not short either; there's a good 40 hours of gameplay in it and probably another 5-10 if you want to 100% it. Concept, for people who don't know, is basically an adventure game with a ton of exploration and questing. You're Amaterasu and are attempting to purify the land of evil.
It's sort of a Japanese game in the sense that God of War was Greek... but if God of War actually had made any attempt whatsoever of being accurate to its source material. I'm not a Shinto expert but from what I do know of the tradition (more than I know about ancient Greece at any rate), the game seems quite true to it's material. The emphasis on purification ritual is obvious, and it's a nice adaptation.
The graphics are, of course, beautiful. It's one of the most eye-pleasing games I've seen ever, but it's hardly realistic (meaning if your concept of good visual is Gears of War's extremely realistic three color pallet, then disregard.) The only true fault that I can find with the game is that you have to read an item description every time you find one; if I've found 900 holy bones already why do you have to tell me that it restores health? In any case, a minor complaint.
-Wii Differences-
As far as I can tell the only real difference is the brush, but considering how much you use the thing, it's a large one. The remote-laser paintbrush adds a lot of good changes not present on the ps2 version, and it controls like a dream, basically the same way re4 was enhanced by changing from analog to laser. You can also alter the size of the brushstrokes easily by moving a bit forward or back from the screen, letting you alter the strength of the brush techniques very easily.
You also dodge by swinging the nunchuck and attack by swinging the remote.
Attacking works well; dodging in a particular direction is dicey, but I normally just jump to avoid attacks anyway. Other than dodging the control is precise and extremely intuitive.