After having several issues with TapJoy not paying off, I have started only doing the games and only on double credit weekends.
For the big payout games I only do ones anymore that meet 2 requirements:
1) They must include a small reward for downloading/interacting with the game. This way you'll have a better idea whether they are going to pay off in the end or not. I've done several over the last few months that didn't offer this, and I ended up playing for a week to reach the first required level, only to find out they wouldn't pay because "download could not be attributed to TapJoy" or similar nonsense.
2) They must pay out a good return on credits for in-app purchases. The really big rewards are nearly impossible for me to complete in the necessary timeframe. But if I can find one that pays out double for in-app purchases that makes it OK to me.
I just started one yesterday, Frost-something, and it already paid out 200 credits just for downloading and opening it. Then today I got to Level 6 already which instantly paid out another 388 credits. Now that it has paid out twice, I don't feel so nervous about making the in-app purchases, which pay out quite nicely - $9.99 = 6516 credits, $19.99 = 16052 credits, so for a few hours of time and $29.98 I'll "hopefully" get 23156 credits, which is about 7.5 times more than I'd get for just spending the $30 through WGT.