I loved portal 1, and portal 2 was good, more intresting but in the end I just got so fed up by the end of it. Sure I still completed the game but if some of the stuff nearer the end just wasn't there it would of made the game more enjoyable and less ''God damn it, not this again'' kind of moments.
Haven't played Co Op yet, none of my friends want to buy portal 2

Worth the money? Just about.
At least one thing valve are good at, is getting a extremly well polished game out on release time.