I switched to Octopus from E.ON last September. Can't say I've noticed my bill's gone down particularly (electric only, I'm not on mains gas) but I did get a free £50 deposit by clicking on a referral link (which they're still doing), & I've still got that as a credit on my bill.
I send them a reading on their website every month (which I could never be bothered to do with E.ON), it's so easy, & get an e-bill pretty much straightaway. Overall, I'm glad I switched, didn't like E.ON particularly (though can't actually remember why !).