My usage - I'm a freelance small business owner (software) who finds the typical personal contract a little too much. The 3Gs was my first PAYG (since the first ever Vodafone PAYG phone) and I've been on contract otherwise.
I use data for all the usual things that most people would - web, push mail & calendars, google apps, twitter, evernote (with images, files and audio), dropbox, RSS reader, etc...
But then I sometimes (rarely) also use additional data - sometimes maybe to bring up a YouTube clip that I want to share, or AirVideo to play video from my home PC.
Quite often, about once every week to ten days I need to spend a few minutes using TeamViewer/VNC/RDP to remote control either my PC at home (usually to retrieve and email files) or another machine.
I expect that 250mb/mo on Orange will be too low although I haven't been monitoring my usage over the past year (can O2 tell me what my usage has been over the past 12 months?), so I have to guess. But I guess 250mb is too low.