I've tried 02, Orange and T Mobile. Coverage is very variable, so in some places one works better than the other. Vodafone has the best quality, providing you can get the signal. I gave up in the end, because it didn't matter which one I chose, each had it's own problems - quality, holding signal, coverage - and was generally a pain.
A colleague at work has signed up to 'The Cloud' and finds that very good. It's a WiFi network, not a 3G system. There are subscription plans (6.99/month unlimited) and PAYG plans. More and more hotspots are coming on line, and he says that it works well. Not very helpful if you want it at home, however! HTH
