I just renewed with o2. It wasn't the best deal or best phone in the world, but it was easy to stay with them, and other networks are poor for signal where I live.
I have a Sony Ericsson K810i with 500 mins and 500 txs for £30/mnth.
Most inportantly though, it's a
12 MONTH CONTRACT. They push you so hard to go for an 18 month, but I insist on sticking with 12, because the amount you can sell your phone for after that period is still decent. The phone is the expensive part for them, which is why all the companies are pushing 18 months. Don't do it!