I also use Quidco, I've had some decent paybacks but you have to be aware they by no means guarantee you will receive the cash back, it depends entirely on whether the transaction tracks correctly and the vendor decides to pay Quidco. I have had issue with one transaction that has been waiting now for 4 months, I think I'll get it, but you do need patience too, as many vendors take up to 3 months to pay out. Alos you need to ascertain if the price - cashback via Quidco is the best deal available, sometimes I find better deals without the cash back. All in all, not a bad thing to use but, like anything else, it needs to be used wisely.
. . . and couldn't see how Privilege Insurance knew I had been through the cash back site, so I ended up phoning them and getting such a great price I didn't have the heart to take any more discount off them.
Martin Lewis seems to recommend them - that man appears to be able to do no wrong these days.