My boys 7 and9, have minecraft on the family ipad.
For a while we had the free version and we worried about their terrible strops and screaming fits when we told them times up. I mean holding on to the ipad so that it was in danger of flying across the room if one of us let go!
I then discovered that with the free version, every time you log off, you lose all the work you've done. So that week the fiver for minecraft was their joint incentive (usually its a couple of quid to save up for stuff in the game exchange).
Since we got the paid version they have been very well behaved. They know they can only go on it when all homework, music practice and their jobs (just little things like setting the table), and if I give them 5 mins warning of the end of the session they are fine about coming off as everything is saved.
They are building a joint 'world' and take turns with it to build stuff. They interact and discuss what they are going to do on it, far more than with other computer games or watching tv. The older one will often do a 'plan' on paper of what he is going to do when its his turn next.
They also love to watch videos of other people's worlds on youtube. Altogether,, out of the various games and other entertainments we have, I would say its comparable to building things with actual lego but FAR CHEAPER than buying a crate of lego
.
We tried to keep our ds's off computers etc for as long as we could, but they are a part of life now and as normal for kids as a skipping rope was for me.