The boss battles, though few in number, truly showcase the teamwork between the two characters. A giant crocodile and a huge anaconda are particularly enjoyable fights, and require a fair amount of coordination and cooperation to complete successfully. Thankfully, the game is quite forgiving when it comes to making mistakes; players share a pool of health, so there's no waiting for your partner to respawn. You're both playing together the entire time. The only section where the co-op stumbles is the airplane levels, in which player two has no control over flight, but is instead a second gunner. Thankfully, this arrangement is used very little, only briefly at the beginning, yet more extensively during the finale.
While Up is generally a very good experience, there are a few problems that drag it down a bit. Most annoying are the dog packs which you have to fight repeatedly. It is very difficult to ge the timing just right to block the dog's strike, since the animation for the block is so brief. This problem is compounded when there are multiple dogs to defeat. It's the only really frustrating part of the game, but these pack fights are reused often so you'll really be tired of them by the time the credits roll.
And it won't really be long before that happens. Up is a very short game, even by movie tie-in standards. I would estimate we spent no more than five hours total playing through the story. There is little incentive to replay, as most achievements are unlocked the first time through. A few non-co-op multiplayer modes, all based on the airplane fights, add somewhat to the longevity of the game, but it's still very, very short. I'd have balked at paying full price for the game a year ago, but now it's under $20 most places, and it's worth that, or at least a rental.
Up: The Video Game is a surprisingly good title. I was quite impressed by the high level of co-op between Carl and Russell. They were a good team in the movie, and that was readily apparent in the game as well. There are lots of serious, more mature games out there that could learn a thing or two from the co-op in Up. It's one of the best games I can think of for an older gamer to play with a child; after all, it's a game about a grown up and a kid teaming up for an adventure! With a high interest subject, forgiving difficulty, and a whole heap of charm, Up is quite a lot of fun for young and old alike.