Some of the events make you too tired to keep on playing.

User Rating: 3 | Mario & Sonic at the Olympic Games WII
In addition to what I said above, some of the games have bad motion controls. The bow and arrow is hard to line up and half of my shots just end up going under or over the target. The running is very challenging in terms of how long you have to do it. Pumping your arms up and down that whole time gets to be tiring after a while. Most of the events though have good motion controls and are very fun while they last. Each event will only keep you occupied for about 30 to 40 seconds. A big problem is that you cannot just choose normal events and play them. You have to put together a set of 4 or a set of 8 to play freeplay miltiplayer. If you are looking for a good singleplayer experience, you will not find that here.
Another big problem is that there are no classic Mario of Sonic tunes. All of the music I have heard so far has been new. Though it goes to say that the music fits thee tone of the game.

Overall, this game provides a good multiplayer experience, but there are better games if you are looking for a game with Mario and Sonic in it, or you are looking for a good single player mode.(SUPER SMASH BROS. BRAWL!!!)