I know people have mentioned it in the past, but I always thought I couldn't be bothered, but then I looked into it again and decided to join.
I used it this last few days for M&S, Boots & Argos presents, and I've already got over £30 in my Quidco 'account' -
M&S was 10% cashback this weekend (is now still 5%).
Seriously - if you haven't got it yet, DO IT NOW! Quidco