I am on three and i can tell you that it is amazing. I commute to work, hereford to Birmingham and the only time it cuts out is when it is in hhe tunnels and one other time for 30 seconds through 1 area. This is on a train at 80mph for 95 mins. I am connected for 98% of the time through half of england. Last week i went from Birmingham to Cardiff and its the same, excellent reception. Three might not have a 2g network, but their coverage in my experience is better than O2 with their mixed 3G and edge network put together.
Not to mention cheaper, i am paying £10 for 15G of data. That's 300mb a day!