It is an interesting read, and I have read several of his books. One thing that should be considered is that wearers of reps are not chosen by random. There is a strong selection bias. Random studies intentionally remove selection bias; but the real world does not.
For most here wearing reps is much more of a game (I use game in the traditional sense, not in the game theory definition). They failed to account for that. I can think of a few ways to have made the wearing of the reps to have been more playful.
IN all, I think it is an interesting study; but I feel they left out an important part and tainted their study. If the question was, "what would a random person do if they were given a rep to wear, and were than paid to wear it," then they ave a reasonable test. However, if the question they are trying to answer is "what would a person who chooses to wear reps do?" then I think it is a flawed test due to, oddly enough, the lack of selection bias in the sample.