I don't doubt that some people have bad experiences with Three (and I know their coverage was awful when they started). Nevertheless, I switched to them mainly because of the positive feedback about them on NSC, although GiffGaff do seem the most highly recommended on here.
The consensus of NSC is that 3 and Giffgaff are the best mobile phone operators. Giffgaff offer the best deals while 3 are close behind while also offering hope of data reception at the Amex.