AVOID
I signed up to 3 on iPhone 4 launch day and I really regret the decision. My signal is poor, I'm based in Uxbridge (West Greater London) I'm forever getting the "network lost" notification and the delay between switching from 3G and 2G is appalling. I'll also loose my 3G signal randomly despite the fact I can reactivate it by searching for cariers and selecting "3 (3G)" instead of just "3".
3 also filter a lot of internet websites which they class as inappropriate (adult themed, feature violence etc.) You have to pay £5 a month to turn off the filtering. MADNESS.
Customer service is VERY poor, I've given up already.
AVOID
Wow!. My experience is very different.
I moved from O2. I was unhappy that O2 reduced the data allowance without changing prices.
I found O2 3G reception to be patchy. With O2, I had no 3G at my house in Sheffield. I often got "phantom 3G". I was seeing two bars of 3G - but when I try to use it, the line degrades to GPRS. So I had had enough of O2.
As I understand it, in terms of network coverage, Three has the largest 3G network. I can now get 3G everywhere, including my house. I can get streaming audio in moving vehicles, on the motorway. I even was able to listen to streamed audio in a department store elevator. Something I could never do with O2.
Three has twice the data allowance - for £5 a month less. (6x if you pay the same!)
And it comes with with free tethering. Which O2 want how much for?
I just got tethering working with my iPad, thanks to iTether.
The content filter is (I agree) completely ridiculous. So I got it removed. After a lot of bitching, they removed it. I suggest going to their blog site to complain, and not to the stores or to customer services.
C.