I'm getting it foremost for the single-player - just myself, on my own, solving some nice puzzles and soaking in the story. I'm thinking that it will be quite different to the original Portal - more scripted, and more in-line with the Half-Life universe, but still brilliant and just as good, if not better.
The co-op is... meh. I can understand that it'd be a lot of fun and that Valve are focussing on it as a feature because co-op is all the rage now, of course... but it's just not what I'm buying the game for.