I recently switched from T-Mobile to O2, and if it wasn't for the fact that I got a really cheap deal on O2 for the SGS2, I'd be regretting it. My signal at home is worse now whereas on T-Mobile it was always full, and my signal at work on 3G is slightly worse; T-Mobile's wasn't great either but it was at least stable for internet use, O2's is rather temperamental.
But as said, it entirely depends on where you are.
As for other issues like billing, customer services etc, they're all as bad as each other. I was with Vodafone for a few years and they got things wrong several times, but I've had mistakes from the others too.