In fairness they also show up on uswitch.com so if they were dodgy I doubt they'd be on there. I got my old S3 through them years ago and it was here the next day no issues. Carphone warehouse or other high street shops couldn't get close to the deal they gave.
I've since sacked off contracts and bought a phone outright, don't like the idea of being tied in for 2 years me. If your not bothered about the latest phones you can get some good deals on refurbished handsets and then get a sim only deal. I pay £7.50 a month with GiffGaff which gives me enough for my usage, they also have other packages if you use more data/minutes. Worth looking at for sure.