Thing with these deals is that o2 won't lose one account each time, they are likely to lose several, and if Vodafone price it right (competitive given it wants market share at the moment) then o2 and others could start to see the power of the resurgent Vodafone.
That said, whilst o2 customer service has dropped down the scale of late, it has a LONG LONG LONG way to drop before it finds Vodafone CS