How to download the Portal 2 DLC: Peer Review on PC

This was a bit of a misunderstanding. The 'skin' you receive from the potato sack is the flag for the game you played the most. For example, I got a BIT.TRIP.BEAT flag. This is weird, seeing as Portal 2 supports actual skins, but this is what Valve said.

Yes, the progress is automatically saved and synced to the Steam Cloud individually - so when you next join a co-op game it will have stored which levels you have completed. This also means that your progress is shared across any computer you play Portal 2 on.

It's worth noting that progress is never saved during a test chamber though, only upon successful completion.

You will be able to pick which level to start at next time you start the game - if you go to the specific chapter you want to play and use the buttons until you've selected the level you want, then go into the assembly machine as usual.