I was on 3 and it was fine for the most part... got good signal etc. What I hated was the customer service people. It was terrible.
I love O2 and would recommend them to anyone. 
3 mobile - boring question Page 2
Quote: RubyMae - Glamourous Snowdrop at Large @ November 28 2008, 3:15 PM GMTI am on pay as you go. It's actually not worth me having a contract.
Also.
Virgin Mobile.
Now O2 actually are based in Slough. Instant distrust if you know what's good for you.
Of course, they are my ISP... :/
I've been on 3 for about four years now and never really had any problems. Worst was when they were setting up and their signal was shit, particularly back up in Yorkshire at my parents. That said, they sorted it out and it seems to be fine everywhere I go.
I do tend to not go for the free stuff though and just go to a '3' shop for an upgrade.
I have got one of the football packages, so text alerts for Bradford City are free (not that there are many of those!) and free Premier League video highlights (which is handy if I've missed MOTD for any reason).
Phones-wise, I'd go with one of their Sony Ericsson's as any phone that is not an SE, I've had trouble with (regardless of supplier), but that's probably a different experience for every person you speak to.
Hope that's helpful in some way or other.
Dan