Hard to tell
I got a SIM only deal with orange about 8 weeks ago. Based in Nottingham city centre and had very little data access despite having 4 or 5 bars with 3G coverage. Luckily I was on a 30 day contract and so binned them off and moved to another network. But other users I know who are still on Orange are still having the same problem (although not quite so bad now).
Phoned Orange customer service and their advice was "if you want to use data on your phone, turn off 3G on your handset". This advice was shortly preceeded by "Orange has the best 3G coverage".
It appears that Orange may have the best 3G _coverage_ (ie you can get a signal), but they don't have the best 3G _capacity_ (ie being able to actually send/receive data).
