from what I've seen an heard, they promise a lot, but often you can waste your time, only to know that some people will be selected for bla bla bla.
That is very true that most of them waste your time...I only do the ones that actually pay and only if the pay is worth my time.
These are just some of the ones i do that i can think of now:
Stanford behavioral lab (only the paying ones)
Pinecone (pays $3 for each one via paypal or check usually the next day)
UCLA Anderson Behavioral Lab (only the paying ones)
Surveyspot (only the paying ones and if they are worth it)
Datatelligence (only if they are worth it)
E-rewards (i redeem them for Priority club points)
Im also registered with a group called fieldworks Chicago which calls me from time to time to participate in focus groups (in person). The last one i did paid me $85 cash to sit in a group for an hour. Was supposed to do another one where they were going to pay me $150 to taste beer...I ended up not doing it for kashrus reasons.