Fairly easy, but v. beautiful to look at and great fun to play. I'm glad I bought it!
I particularly like the way both Carl and Russell are constantly part of the game, in a similar way to how Sonic and Tails were always together in the classic Sonic 2 game from aeons ago. But here you can switch between Carl and Russell as playable characters, and indeed you need to quite often because each character has different abilities, and certain challenges require one or the other's abilities to complete them successfully. It adds a real logical thinking element to what is already a wondrous platforming/exploring game.
If you like Pixar's movies, I recommend you to get this game because it is by far the best one linked to their movies. If you don't like Pixar's movies, I still recommend you to get this game because this is how a movie tie-in game should be - pure class!
A final quick thought: I've got the PS2 version and the graphics are very fine, as good as I've ever seen on the dear old machine. But I've seen it running on a PS3 and a Wii in my local gaming shop and the graphics on those machines are so incredibly good that the game looks like the actual movie!!