My work changed our phones from O2 to EE (Orange/T-Mobile) for financial reasons and it was possibly the worst service we've ever encountered.
Coverage was either awful or non existent, and if you were fortunate enough to make a call it would more often than not cut out.
There were never any issues at all with O2 coverage, and i've always been with them myself, plus their customer service people have always been very helpful, even though i haven't needed to speak to them for years now.
In comparison i tried Vodafone once a few years back and lasted 20 hours with them, and 4 of those were on the phone to their very rude customer service people, who had a habit of passing you on to anyone and everyone, then cutting you off.
Every time i asked them to cancel my contract they just refused point blank and gave me a load of bullshit, so i went back to the shop where i took the contract out the day before and the very person who happily took all my details told me "Sorry, we can't cancel contracts, you need to speak to customer services."
After a lot of abuse at one person on the phone, he eventually crumbled and cancelled it, then i humbly phoned O2 asking to go back with them and they said "Don't worry, we'll pretend you never left us."
Nice people.