Unfortunately, I'm fairly certain the Motion Plus only works for new games that support it.  If you play an older game the attachment, nothing is different.

That aside it's not bad at all on the Wii, it's just the learning curve is much steeper.  I've only owned it and played it on the Wii, and I don't have trouble doing the movements (now anyways).  It definitely takes awhile to get the hang of it, but the learning curve is well worth it.

Depends on your preference of playing games I guess.

If there's still anyone on the fence (all two of you who haven't bought this game yet), PS2 Okami is now $9.99 for a week at the Capcom store.

I played both the PS2 and the Wii version. And I prefer the PS2 version. I played through the whole game for PS2, and gave up near the beginning for the Wii. The controls are so bad, I gave up on it. Or maybe its because I'm not a very patient person to begin with. I dont mind the graphics for PS2, but I think for this game the controls matters the most. Bad controls can ruin a whole game for me.