It is always fun to earn some money in a way that contributes to science for the good of people, and in the social field it is always nice to meet other volunteers.
Communication is always smooth, personal / telephone and sympathetic. The atmosphere is fun, but you usually decide for yourself of course. Time spent is not too long, depending on the trial you choose. Compensation is certainly sufficient and fair for the time spent.
As long as I can, and want to participate myself, I will participate in such a study every time. Until now, it has always been fun.