I am a parent of three kids, ranging from 6 to 11 years old, and all three of them adore this game - and frankly so do their parents! A couple things not mentioned in commonsensemedia's review that I would like to point out though:
1. There are different ways to play Minecraft. My youngest really likes putting the game into "creative" mode, set on "peaceful," and then there are no zombies, skeletons, spiders or creepers (the main monsters of the game) trying to attack you and you can't "die." It's more like building with legos on the computer. Unlimited access to fun blocky resources such as grass and dirt, stone, and precious gems. All three of my children actually enjoy doing this, and they come up with some amazing creations this way.
2. You never *have* to play this game in a common server where you are exposed to other users that you don't know. We always play on our own computers at home, and sometimes open one computer as a server to the others so that more than one child or adult can share the same world and work together to find resources.
I would suggest checking out some of Paul Sores Jr.'s youtube videos about Minecraft, if you're considering purchasing the game and want to know more about how it works. He has a whole line of tutorials as well as fun stuff like "Punchwood Island" where his whole family gets in on the game. His videos are very safe for all ages, and rather entertaining too.
Happy mining and crafting!