I have been with Orange for 7yrs now as work pay my contract. I have been very unimpressed with their data network, but Im unsure whether that is down to their network or the cheap orange branded WM handsets we get. Orange Firmware is a dog, they remove any functionality from the phone and replace it with the option for their paid apps, so the first thing I tend to do is wipe the phone and install a custom ROM. I currently have a Samsung i780 running a cooked WM6.5, and the web is so slow I dont bother.

One of the reasons I have bought this phone is for the web experience, so Im going to try Orange for a couple of days, if I dont like it I have a PAC code which I will switch to t-mobile.
Onde thing Orange are though is strict with their data limits, if you go over your tarrif, you will pay the higher data charges, they dont offer the same flexibility other carriers do..