O2 - no coverage,high latency, no throughput. But good fake numbers ;)
Ditto all the coverage comments - O2 have the worst coverage of all, I carry a "3" SIM for emergencies.
The speed claims don't match what I see. Out here in the suburbs I get good 3.5G and high *sustained* throughputs. But the latency is so poor browsing is near unusable and the image compression is shocking. In the city I get great signal strength, great rated connection speed but no actual throughput at all for extended periods of time.
At the best of times the latency on O2 is terrible and seriously harms browsing, at worst - well I've seen 30second latency occasionally and 5-10s is fairly common in the city (why throughput is so poor). I've had browsing sessions run faster on 2G than 3G at times, thanks to that shitty 3G latency.
What they measured has fsck all to do with what users actual need.
If O2's bastard child giffgaff weren't so damn cheap I'd be on "any other network". If I actually needed my phone I'd move anyway - luckily I work at home and my normal pub crawl has continuous Wifi coverage ;)