I've changed suppliers twice now (not including insurance), once for gas+electricity and once from virgin to sky/bt.
Both times there is no problem in setting up the new accounts, nor indeed in letting the old company know you are leaving them. Both sides assured me that it would be cancelled by x-date and transferred smoothly, blah de blah.
Yet - both times it got cocked up.
First time I ended up paying two companies for months for my utility bills and Virgin, well Virgin, the little darlings continue to bill me for my phone line rental despite personally asking three times for them to stop. I have been given 100% assuarance that the matter is dealt with now

I've just been writing up some pages on utility prices and have been looking at uswitch etc and happened to do a search today and could, in theory, save about £240 on my current gas+electricity bill. But that quickly becomes a mute point if you spend hours on the phone getting annoyed because it hasn't gone as smoothly as you hoped.
I realise by asking this I will get more pee'd people than not, but what are others experiences in changing suppliers and did you save as much as you hoped?