I had some time to kill a couple of years ago and signed up to two of these survey companies. One required £30 to cash out and they stopped emailing at £28. The other paid out at £5 but paypal said their link was invalid. Emails to those companies went answered on the latter whilst the first one said they in effect didn't believe my answers so the 'contract' we had was terminated.
I too have heard stories where individuals have received nothing or emails have stopped just short of paying out.
My tip is STEER CLEAR OF SURVEY PANELS/COMPANIES.