Been with them for a couple of years now and had no problems really, was with EE and used to get amazingly fast 4G speeds of 1mbit at home compared to 50mbit plus on Vodafone.
It was not even getting 3G when walking down the main shopping street in central Newcastle, they were pretty good out in the sticks actually, it was in built up areas where i spend 95% of my time they were worse than useless. That's before I had the misfortune of having to deal with their customer services where they charged me the wrong amount for about 6 months straight.