I shared my steam library with my little brother yesterday. I almost forgot how fun it is to backseat play Kerbal Space Program.

@Nanos 1) game state is separate, the second player gets a warning that they may force the first player offline and if they confirm that the first player goes into offline mode. The game doesn't necessarily stop but all Steam integration is gracefully stopped. This foregoes abuse by doing exactly what a sophisticated exploit can achieve. The assumption is that the loaner DMs the sharer if they can use the game, and if the loaner kicks the sharer off without consultation the sharer can just cancel family sharing. There's a base level of trust in the system.

@Nanos 2) KSP is fun but it's very time consuming. Even in sandbox mode it requires countless trials to get anything done, and if you fail you have to start over. The modding community is amazing though. It's my favourite game of all time.