I actually hadn't heard anything about this game, until I happened to be in the video store looking for a different game to rent. The original game I was looking for was nowhere to be found, but I happened to see Mario and Sonic at the Olympic Games, and thought it looked like fun. If nothing else, I figured that my kids will probably like it (both being Mario AND Sonic fans).
It turned out to be TONS of fun. We had it over Thanksgiving weekend, with all of the extended family, and we had an absolute ball.
At first we thought the controls would be WAY too hard to use, but after the first event or two, everybody pretty much had the hang of it.
I've read in some other reviews that people don't like how the controls are too similar in some of the events. I actually like that some of the events share the similar (or even the same) controls. This is one of those games that you want to get in and play, not spend tons of time memorizing different control schemes for all the events.
That being said, the sensitivity for some of the events is a little odd at times (long jump and triple jump come to mind) as it seems to occasionally ignore your actions. I think this is actually due to not having the timing correct (i.e. lifting the Wii remote too soon to jump).
The sound is and music is about what you'd expect from a Mario or Sonic game. Nothing too fancy, but done well enough to not be annoying.
The voice acting is about what you'd expect. A few one-liners for each character, but it's actually done fairly well, and actually got me to laugh a few times. There isn't a ton of it, so I couldn't justify scoring it much higher.
The graphics look great, and go well with the cartoony theme. The subtle animations are great as well (Peach twists her hips as she claps, etc), and add a little life and personality to the characters.
The replayability for Mario and Sonic at the Olympic Games is fairly high as well, due to the great multiplayer, as well as the ability to win different trophies for each of the circuits, AND (my favorite) the ability to set Olympic and world records (assuming you can beat the existing ones). You can also go online and see how you compare to others playing the game.
The fact that it kept my kids busy for 4 days straight (they're now asking us to buy it), and that it got so much use by the whole family, and the laughs (watching people as they get their character to run as fast as they can is absolutely priceless), make this a game that is definitely worth owning.
Final Verdict: 8/10