Just switched from British Gas TO e.on for both gas and electricity.
They're helpful on the 'phone (use saynoto0870 web site to find a free number), but it was a bit of a palaver getting it organised. They're threatening to save us between £250 and £650 p.a., so worth it. All the on-line comparison sites said they were the cheapest (for our rate of consumption, your mileage, etc.), although the actual calculated savings amount varied wildly.
I'm delighted to be shot of British Gas though: complacent, expensive, have idiots in their marketing department, and are very hard to deal with.
Cheers,
E.