I’ve been through Portal 2 co-op twice, mostly with my friend Chris.  I’d say it’s definitely replayable… except on the last set of tests, we generally didn’t remember the solutions, only the puzzle.  (“OK, I remember this room.  You did a thing, and then we had to do something else.”)  I doubt it’s massively replayable though… like, even if I had the money for it, I wouldn’t buy stuff to outfit my robots.

Jump now! No, now!

The co-op games are an entirely different set of levels, and they’re very cleverly designed to require, well, actual co-operation.  I can’t imagine it working well with a random pubbie, or with someone who’s been through the puzzles and is impatient.  But it’s a blast if you’re figuring it out together.  As a point of pride, I never killed my partner on purpose.  It’s more hilarious when it’s by accident.
It’s interesting to see different play styles.  I think Chris likes to look around a bit.  I immediately put portals on flinger targets and paint dispensers, just to see where they go.
There are some tasks you do for GlaDOS, but the story is quite minimal, and as the bots are pretty silly, you don’t get very invested in them.  It’s more of a pure puzzle experience.  (Though you do get a big additional dose of GlaDOS’s particular acerbicity.)
I generally like co-op games (L4D2, GTA IV, and Borderlands all have good co-op modes), and this is about the co-oppiest, as both partners are needed.  I sure wouldn’t be bothered if Valve created another five sets of tests…
Edit: Chris’s notes here.  If you are ever locked up in a test facility by a mad AI, Chris is definitely someone you’d want trapped with you.