Are paid to answer surveys real?

I have been really successful with a survey site that I have been using now since February 2010. I have been consistently receiving a check every single week. As for sites that say they will pay you a set amount for you to take surveys there is something wrong with those sites. Figuratively speaking marketing researchers look of certain demographics on various opinions of various products so how can a site guarantee what surveys will be available and if you would qualify as they do not know what the researchers are looking for until the surveys come out. I would not entertain a site that says they can guarantee you a set amount as although I have been consistent with my survey site, there are surveys I do not qualify for, at times.