O2 have yet to charge me or use my allowance up for a single mb of web browing or email GPRS use.
Good use actually, means I can email any photos I take to my email addy.
Their upgrade system was good, infact I could've upgraded two months earlier if I'd wanted (I now have a 6630).
Their online customer care form is excellent and I've had a reply several times in the first 48 hours if not 24 or even 12 hours.
O2 all the way.