While it's very nice to your users to replace CAPTCHAs with something more fun, it isn't necessarily more effective against attacks or spamming. Classic CAPTCHAs can be broken by simply using cheap manual labor (working slaves in low-paid countries, or willing surfers who gain access to porn sites by solving some CAPTCHAs). Also, those games might be ineffective if there are too few combinations. In one of the examples, the user has to put 2 of 5 items into a box. There are only 2^5 = 32 possible combinations. When trying to break this game by robot/software, using a random guess, you succeed every 32th time. That's still enough for spamming!