I have been with o2 for a long time now ( In fact from when it was Martin Dawes ), and I have never had a problem with them. Change my contract, not a problem. Change my phone, not a problem. Change my number, not a problem. Etc etc etc.
I also have the broadband and now the home phone and again...NOT A PROBLEM!
The reason why i'm telling you this is because I have read alot of threads on this forum and a vast amount of them have been about how bad o2 is. But the majority of problems have actually been caused by you the consumer.
Don't get me wrong, the people that work for o2 are not perfect, but please realise if you give them the correct information and ask the right questions you will have a better chance of things working out for you.
Also get some knowledge before you do something to your phone, or ask about tariffs, etc. It will help!
Good luck to you all.