I skimmed the user agreement a bit, as one should do.
You must be 16 years of age or older to participate in ClixSense.com
You are solely responsible for the reporting and payment of any taxes for money earned from ClixSense.com
agree that ClixSense.com may transfer and disclose to third parties personally identifiable information about you
This is about all the catches I could find. I really don't think it's a scam, but it's not tax-free money, so i guess you are frauding the state if you don't pay taxes on it. And I guess the age thing can be trespassed without any major trouble. And they can send information to third party companies. This means that you might end up getting spams in your inbox.
Addment: I higly suggest taking a look in the user agreement. Because when you press that button, you are agreeing to a whole lot of things.