I went from Virgin to GiffGaff because Virgin can't provide a 4G service. I'm pleased with it, but as there is no contract you are not tied if you don't like it.
I always by my phones outright, unlocked so that I can get a SIM only deal from wherever I like.
GiffGaff works out cheaper for me and gives me 4G which Virgin can't do. Seems daft buying a top of the range 4G phone and be limited to 3G.