It's a very fun, simple game. It's just so horrible being stuck on one puzzle for the longest time because one's brain tends to get tired and lose interest. But solving the puzzle also gives us that "ah ha!" moment too. xD

And you're right! My roommate and I finished Co Op for Portal 2 and it was so sad that it had to END! I mean, we were rushing to see what happened next, but dreading the moment it would finish. Games are just so fun sometimes.