When I was with British Gas they were the pits. They never read my meter, sent out "guestamates" with wildly inflated prices. When I phoned them with the correct readings, they'd send out an adjusted bill based on the spurious one which was impossible to understand. When I moved to Eon (Powergen at the time) BG phoned me up asking why I wanted to leave them! When I said they never read my meter they stated that they had, by law, to read the meter every two years or something. I asked them to check when they had last physically read the meter they had exceeded their time limit by something like two and a half years.
I like Eon's web site. I'm on a fixed tariff. Every time a new tariff is brought in they email me stating there is a new tariff available and will state if I'm better off staying put or changing. They tell me the meter man will be calling soon to read the meter. I then send my own readings irrespective of whether the guy calls or not.
One thing I've noticed with the "Switching Sites", if you enter for example that you are with "BG on Moonbeam Tariff" they might suggest "Eon on Sunspot Tafiff". Close the page, go back and enter that you are on "Eon/Sunspot" and they will almost certainly suggest another tariff, possibly "BG/Moonbeam", so I have little trust in those sites.