I have been taking this paid surveys thing for some time and have found out that the legitimate market research companies really do pay. If you are into it, you can earn like 100 to 200 $ per month. But they are not going to make you rich. Also never pay to join a survey site. They just give you a list of survey sites which you can otherwise find free.
There have never been any security problems. Most of them don't sent any email except the online survey invitations. You may qualify for only some of the surveys sent to you.
Some of the recommended free survey sites are: Surveysavvy, brand institute, NPDOR, Greenfield, Mysurvey, Synovate, Lightspeed, ipsos, pinecone, Your2cents, surveydirect