Orange are OK when you go over your 1gb limit, obviously you need a program like Bandwidth Monitor Pro to keep tabs of all your uploads, downloads. You will receive a warning email if you go over, but this is simply an advice email.
Out of the four air cards I have, this would genuinely be the one I would recommend, mainly because of the UK coverage that Orange have achieved.
Despite criticisms on their service, I have always found getting through to Orange not bad at all, and they are always very helpful.

I have had both a Vodafone and Orange datacard trial now, and I have found the Orange Card to be much better, faster and easier to use. I think the Vodafone tariffs might be a tad cheaper, but overall the coverage for Orange is fantastic. I have used the card whilst on the train, in a car, at home, in the office, in a field, everywhere. It's brilliant, I'd never go back!

Installation of Orange Mobile Office 3G is straightforwards and the new software/firmware is much improved but it can still cause some problems (eg occasionally it refuses to start before a reboot) and its overly fancy. Why can't it just connect without having to launch a browser? It has the best 3G coverage in the UK and the speeds are impressive 200 KBs + once it gets going. There is some latency but that is a 3G problem known as cell breathing.

I popped an Orange Mobile Office 3G data card in my laptop and simply can't fault it. My laptop is now free to roam wherever it likes and I can check my emails anytime. Not tried out the competition so can't comment in that respect, but this little beauty does everything I wanted it to.