In my experience the earnings are poor and you could end up on many spammers' lists. Often you need to do a number of surveys and reach a threshold before you get any money and it's easy to lose interest before you get any money. Sometimes the rewards are useless vouchers such as £10 off if you spend £50 on wine somewhere. I do have one that sends me a £1 Topman voucher though every time I do a survey which takes 10-15 minutes, which is just about worthwhile if you have nothing better to do.
Another problem is the surveys can be really long and then at some point you are told you don't meet the criteria and the survey is aborted before the end so you get nothing.
Others may have had better experiences.