I had a blast playing it with a friend just now. There are 20 new levels, this time designed for co-operation mode in Portal 2.
One of the players controls blue/orange portal, while the other player controls the green portal. The green portal allows you both to travel between past and present. Causality matters - placing an object in the past causes it to appear in the present, so you have to think in four dimensions, just like on Portal Reloaded's singleplayer campaign.
I highly recommend it. Mind-bending and fun for 2-3 hours.