My wife and I have played London 2012 for a few days and, having gone through all events, we can say that it's a really nice Olympics game. We liked the variety of available sports (we haven't played Beijing or Athens to compare), and having things like shooting, keirin cycling, weightlifting, kayak and trampoline was a very nice thing. We also appreciated a lot the effort put in more "complex" sports like beach volleyball and table tennis.
We grew up used to the button-mashing system and were happy to see that London 2012 dropped it almost entirely (only weightlifting requires the "classic" button-mashing). In the rest of the sports, most of the instructions are designed to replicate the body movements of the athlete. As a previous reviewer said, some sports are easy to learn and hard to master. Some other ones are easier (I broke the high jump world record in my first attempt playing it), but to me it looks like one may never reach the point of being able to repeat the perfect movement every time.
Athletes names and faces aren't the real ones, but you can fix it through customization. It will take a lot of your time, but if you want to run against Bolt, swim against Phelps and play volleyball against May and Walsh, you can do so. We are customizing our game slowly.
We also appreciate the chance of competing in sports where everyone plays together, and not only the "only one player per turn" system seen in most Winter sports. And we understand that it limits the number of players to 4, since it works with a split screen and splitting it in 6 or 8 parts would create a very small image.
In the end, London 2012 seems to be a nice game to call your friends and play at home (even better if you have 4 controllers to allow more competition). But, that said, there are some minor flaws that prevent me from awarding it a full 5 stars: lack of victory ceremonies in "event" mode; national anthems cut in half; small number of countries compared to the number of nations in the Olympics; the "points" system in the medal table, which no one uses as far as I know. As you can see, none of these are related to game playability; these are just details that could be improved, although I don't know how much memory and work would be necessary to have, for example, all 200-something countries (each one with its own uniform, athlete names and national anthem).