The World of Goo is challenging and extremely enjoyable. Everyone in our family loves the game and has gone so far as to get the game on several different systems in the house.

Even if you don't really like puzzle games, you'll find yourself returning to this over and over again to get past that last level.
It's very challenging and, as a homeschooler, I use it as "down-time" between classes for our kids.

Try the demo. You're gonna love it!