It's not bad.
Graphics: The graphics are actually pretty good, for the Wii. Everything is detailed and looks real. But there are a few graphic glitches here and there. 9.5/10
Difficulty: The only hard part about the game is that the controls can be a bit frustrating at times, but at others, they are responsive and work great. I always have a problem with the Javelin throw, though... 7/10
Story: This is where things get a little weird. Why would the Sonic and Mario characters want to enter the olympics? Hmm... to rub it in each other's faces? It's a little weird. 6.5/10
Controls: I had already mentioned the fact that the controls can be in the way at times, and they can be a bit finnicky, but they work pretty good. Not bad. 7.5/10
Overall, it's a fun little party game that can be fun at times. It's not very fun if you play by yourself. Invite a friend over to play it with you. I'd say rent it first to see if it's up your alley. 7/10.