Which is better for pay as you go. I'm with T-mobile but I think that they charge too much to make calls. Is it worth switching to O2 or not? Or are there any better networks?
All opinion welcomed.

O2 are a great network, very in-touch with the user.
Also they have great deals involving free texts and calls to pay-as-you-go mobiles.

o2 is the company who allowed someone to take out a contract in my name fraudlantly and then promptly billed me for it expecting me to pay for it when I had no idea of what happened as I have never used O2 at any point in my life.
Their administration is very poor and It took me months to solve the situation and I even had to get the police involved as O2 allowed someone to use my name WITHOUT ID being present.

I have been with T Mobile for years I have had no trouble. As for charges maybe you should go to Carphone Warehouse etc and ask to see your options, there might be a cheaper deal for you.

DO NOT SWITCH TO O2.