Have used these guys 3 times now - all cashback deals. Have to say they have never skipped a beat on the cashback. As long as you send the correct docs and in good time (I schedule it on my phone) they always text me to say my claim has been accepted. Cheque arrives 2 weeks later. Done this now probably 16/17 times and never had a problem.
Can't say much about their customer service line because I've never had to use it. They do some great cheap-as-chips deals too. Only 4 stars as they don't provide geographic phone numbers to call in on. A small fault but most others are the same.