I switched to Eon and a few months ago they lowered my dual-fuel bill by £15 a month, and I am still currently £42 in credit. I did a comparison at the start of this year and they were still virtually the best price for my useage.
The swap-over went seamlessly and I got 2500 Tesco points when I swapped over (or was it 25,000? - whatever £25 equates to in points anyway, lol) as the guy that nabbed me was in Tesco at the time.
I swapped due to very poor customer service by Scotish Power who had supplied the house with gas and leccy from new but on 2 seperate accounts. When I moved in a few years ago I rang to ask why I had no dual-fuel discount. They promised to sort it and it would show on the next 1/4 bill. After ringing every 1/4 as the discount never occured, I got fed up and switched.