A great plugin with huge amounts of customisation! You can create multiple reward systems, combining them, or to make them work separately, creating fun achievements, so your website will eventually turn into a video-game :D (Well, it depends on how far will you go with those rewards, but it is hard to stop).
If you want to create reward system - this is your best choice (both functionality and cost-wise)
Also, the Support Team is very helpful and tries to "fix" anything you have problem with in a very short period of time, even if it is not that easy on their end. And they're just nice to talk with :)