I moved from 02 to 3 and im currently on a 30day sim only deal for £15 per month
Data wise my deal is only supposed to have 1GB, although recently its been doubled to 2GB.
Coverage and signa; wise, it was ok, but over the last year or so things have dramatically improved.
As for customer support, well, "hello and welcome to tree" its based in India, however i have not had much need to use it as the network, and billing does what it should.
If your unsure you could try them with a pay as you go sim before you take the plunge, you can make some savings on a 12 month sim only deal, although for me i much prefer shorter contracts.