I've been with o2 for a couple of months now - only criticism is the Wireless Box iii is crap. When I first got it, I got about 1.9 mbs. Then the router died so I plugged in my old Netgear DG836N as I wasn't keen to be without internet for a few days while they sent a new router out. They changed my profile to ADSL from ADSL2 and the speed went up to 3 mbs. Don't know whether it's the new profile or the router that makes the difference.
Incidentally, I was on a BT wholesale connection prior to o2 and was getting about 1mbs at best. I'm about 3km from the exchange.