I left Vodafone for O2 just to get the iPhone 3G when it launched. I bought a PAYG 3GS last year and sold my 3G. As soon as my 18 months was up I moved back to Vodafone on a rolling 1 month SIM only plan and I couldn't be happier.
I had no problems with O2 but Vodafone have better coverage in Brighton and besides, the technology has moved on during the last 2 years. When I moved to O2 they were trialling femtocells, useful to me because there is no reception from any network in my flat, but they never introduced them. Instead, Vodafone have released their version, the suresignal and it works just as advertised.
Their customer service is also top-notch, in my experience. Even when I asked for my PAC code 2 years ago, I remember them being a model of politeness and efficiency. Only last night I phoned them regarding the iPhone 4 I've reserved on launch day. Their operative knew that I'd need a MicroSIM and arranged to have one sent to me next week.