Minigames are generally designed well and are great in multiplayer.
As well as the main events, there are so-called 'dream events', which are different takes on a particular sport, but done in a way that wouldn't be possible in real life. It's a good idea, but for a lot of the minigames, you can't even tell which sport they're meant to be like, making them feel like mario party minigames or something. Regardless of that, they are a lot of fun, despite being a little too cluttered.
There's also a multiplayer mode that takes place in London, which has you run around in a miniature model of the city. Every so often you play minigames, but a lot of it involves playing in London. You need to do your best there to shift the tide over to you. But this mode is simply lacking in ideas and is tedious to play.
Mario and Sonic at the London 2012 Olympic Games isn't a bad game, but it doesn't do much to distinguish itself from the original.