In short, Peggle is less a game than a delivery mechanism for positive reinforcement. There are 60 "challenges" to complete, but when so much of your success depends on a lucky bounce, you're not really playing for the "challenge," are you? Admit it: You're playing for the rainbows, the fireworks, the ribbons and trophies, the feeling that you are important and good and everything is right in the world.
Bottom line: It's more Peggle. If you liked the first game, you'll like the sequel.
Recommendation: If you need a quick shot of happiness, it's safer than anonymous sex or recreational drugs.