Peer Review is now up on the XBL Marketplace (under Game Addons, Portal 2, not spotlighted yet) 1.37 GB download. Not available on Steam yet, it seems. No idea about PSN either.
Description text says there's both singleplayer and multiplayer challenges.
EDIT: The singleplayer "content" just appears to be time-trial and fewest-portal challenges (no steps challenges this time) for the story levels. Nothing unique.
Similar for the multiplayer, new leaderboards for the original courses, but there's also another series of levels called "Art Therapy".
Took about 2 hours to get through it with a random dude I met on XBL. As always, with Portal, the reason some of the puzzles took so long is because of overthinking the situation. It's a total "Of course!" once you see the proper solution. Pretty decent overall, and you can't argue the price.