Progress in multiplayer is based on a per-player basis. Each time you begin a co-op game with someone you haven't played with before, you'll start from scratch. In one respect, this makes sense. Due to the progressive nature of the challenges, it's not unreasonable to want players to experience them chronologically. That said, this implementation almost guarantees that folks won't be playing a whole lot of co-op with random strangers, unless they simply want to play the first dozen levels over and over again. Once you've beaten a puzzle, the game lets you go back and replay it with the same partner; however, there's not much incentive to do so once the puzzle has already been solved. (I was pleased to note, however, that the PC to PS3 co-op test session I conducted with Shacknews' Steve Watts worked quite well for several levels, until the server crashed.) Portal 2 also includes an in-game store, where players can spend real money on hats and other cosmetic enhancements for their co-op robots, including extra gestures and taunts. Though these knick-knacks only run a buck or two each, I still can't quite get my head around purchasing such stuff for a linear, puzzle-based experience. Unless Valve plans on releasing reasonably-priced co-op level packs at a regular clip (which I'd happily purchase), spending extra cash on virtual swag that only my co-op friends will see seems pretty ridiculous. I had an absolute blast playing through both of Portal 2's campaigns, but I'm not sure that there's much reason for me to play through them again, beyond wanting to re-experience the witty script and impeccable voice-acting. A striking lack of replayability aside, Portal 2 still manages to serve up some of the best solo and cooperative experiences I've had in quite some time, thanks to its great puzzles and truly memorable characters. Provided that your notion of a game's value isn't tied too heavily to the number of hours you spend playing (or replaying) it, I'd highly recommend seeing what Aperture Science has in store for you this time. You monster.
'Whatever you do, don't press that button!'