I have been a member of crowdology for 2-3 years now and since then I have earned 15 pounds. The surveys are limited to a certain number of people, so whenever I get an email, I try to take the survey as soon as possible. I receive a survey invitation about 3-4 times a month.
The survey amounts vary from 10p to over 2 pounds, though personally 1.50 was the highest I was offered. It doesn't take long to fill out the survey and the screening questions are very short, thus you are not wasting 20 minutes answering questions to then be screened out.
The great thing about Crowdology is that it doesn't pay in vouchers but money, which they transfer to your paypal account. Although it took me a while to withdraw the money, eventually it worked without problems and the money was transferred in 2 lots. Having read other user comments it seems the website has technical problems from time to time when you want to withdraw money - I had to keep trying for a few days before the transfer worked, but since then I've not had problems and have been offered more surveys since I withdrew the balance as well.

In short, this is a great survey site if you are fast enough to do the surveys before the maximum number of people is reached.