I have been using O2 payg sim w/ data. With the p1.1 update I just used my free 3 sim for the first time. PAYG 10 pounds for 150mb for 90days, then call 444 for add on and they will give you 1GB/ 30 days for 5pounds of the 10 I just paid leaving 5pounds for call.

But... where I live, N London, the connection keeps getting lost in my house, not a problem with O2, so I may can the 3 sim after I get my money's worth and re-topup my O2.