I'm on giffgaff and think it's great. They even pay you a decent amount of credit (I got about a tenner the other day) for making forum posts, and I've only posted a couple of times. Its also got the lowest payg rates available and free internet till the end of may. They haven't announced what it will be after that but they just announced unlimited texts for a fiver and unlimited text+internet for a tenner, so maybe they will have just internet for a fiver too after may. I know I sound a bit like an advert, but I have no complaints at all.