I can't give an objective statistical answer (or even a first hand subjective answer, as I haven't been a customer of theirs - I left Orange a couple of years before that), but from people who are customers to other random conversations, I've heard lots and lots of people say coverage got better due to the merger, and quite a few say prices got better. Don't think I've heard a single person complain (specifically about the merger - some complain about the company, but that's true of any phone company).

Edit: there's also the formation of Virgin Media, which has been a major force in the UK for 6-7 years now, and is made up of a couple of mergers (NTL + Telewest to form NTL: Telewest, then a few months later a merge with Virgin Mobile). I've no idea if this made it better or cheaper, but certainly I don't remember ever hearing specific complaints against the merger (either when it happened, or afterwards)