I have had a Voda dongle for 3+ years now. While coverage in London and other cities has improved the coverage along the railway lines where I need it most is still next to inexistent.
Out of 45 minutes on Cambridge to Kings Cross for 20 or so there is no coverage at all, 15 are 2 G only and proper 3G coverage for only the remaining 10 or so. Out of 120 minutes on Cambridge to Liverpool street and Cambridge to Peterborough are even worse. East Coast mainline is a similar joke - there is little or no coverage along most of it.
O2 is not any better. They deployed Edge for sake of iPhone mark 1 and stayed there. Their 3G network is nowhere to be seen. I have seen my Nokia E71 on 3G data only a couple of times in the UK while in 5th world ex-soviet block backwater it regularly cruises at the max speed it can do (the lowest notch of HSDPA).
It is spelled MOBILITY for Christ sake. It is there so people can use data while MOBILE. That means covering main routes for public transport so people can use it. I could not care less how many towers you have along the London skyline in augmented reality or not. I do care however how many towers you have along the east coast mainline as on this depend can I get some work done or not.
While 3 has gaps it clearly looks like one of the best options out there at the moment. I am definitely going to cancel the Voda contract and go 3 for my modem. If it is better than O2 or Voda (as I expect) I will move the rest of the mobile data in the household as well.