I-Ninja hates me. I had to replay the third "sneaking" mission in the third world for some 40 or 50 times, no exaggeration. I've been playing it for three hours straight, cursing like a madman, but I finally made it. I believe I still have about one half of the missions to go.
If you think you can take anything a 3D platformer can throw at you, this is the game to try. Basic missions are seductively easy, but those timed monstrosities are HARD.