I have been with three on a sim only plan rolling 1 month as I was not sure what the call signal and internet signal would be like. I have been hugely impressed by both! other people have been complaining about the signal and I do feel for them as it is frustrating, but I can't say I have had an issue and I have traveled to other destinations other than my home in Essex and it's been fine.
Best 3g speed I have had for internet so far is 25meg download, it beat my friends 4g connection on EE!!!
When I went to the store to get the contract the lady was very honest about the signal and without me asking she showed me a map of where I would have signal and where I would not in the locations I go to most, even in the areas marked as weak or no signal I still get it.
Phoned customer service for a couple of things like activating international roaming and checking a bill and can't say I have had an issue there, very on the ball, yes it's an Indian call centre but who cares?
I have been on all the main networks such as vodafone, o2, orange, t-mobile, plus GiffGaff and three is the best, vodafone is terrible and o2 internet speeds are diabolical even with H+.
To reply as a company, please log in to your business account.