After going through about 5 Philips Sonicare brushes (They seem to literally vibrate their own electronics to bits) I switched to an Braun/Oral B non-sonic one and it's been absolutely fine. There's a cheap one which only does slow speed - avoid that and go for the fast one.
My dentist has said I should get an electric toothbrush. There seems to be a big variation in prices for them, what's worth going for?