My mate had one and it worked well. The thing is you have to claim the cashback within small window of a couple of days, and if you missed one then you missed all remaining months worth. The aim being that most people miss one and lose out on a lot of money.
I have extensive experience with them and yes, if you buy it from a proper place it's completely trustworthy (I always go with the carphone warehouse/phones4u or one of their affiliates like dialaphone and e2save - probably many more as well).
Thanks guys, this has made me feel better!
Sometimes it goes wrong like it did for my brother. I'd prefer one that just gave you a cheaper contract monthly end of rather than reimbursing you some of the funds every month to make the difference.