Zack & Wiki does have some tough levels, in my opinion. It's one of those games where some things click right away, and then something obvious takes a while to figure out, and then you're like, "D'oh! Why didn't I think of that sooner!" The good thing about it is that it's pretty logical. Once you figure out a puzzle, it makes sense.
I'm loving it, but it's on hold because of...Galaxy! I just can't play anything but Galaxy, I love it so much.