I’ve just switched to Giffgaff. It runs on the O2 network (so O2 coverage applies) but is community run with support over the forum and self managed via the website.
Pricing is like a contract, but with no tie in at all. Seems good so far.
If you don’t go with it as your primary number, it’s cheap enough to get a SIM only as backup to stick in an unlocked phone so you can cover more bases – say, Vodafone or Orange on contract and a Giffgaff sim to use the O2 network.
You’re not paying for call centres (most of which have been poor to terrible from my experience). Support is via the forum, payments via the website and advertising comes from word of mouth – if you sign up from a referral, you get £5 credit (and so does the referrer)
If you are tempted, you can use my referral link to get a fiver for each of us (or just order off the main site if you want)
http://giffgaff.com/orders/affiliate/gravityslave
You’ll need to top up by £10 to activate the SIM though.