Try the different operators on PAYG or 30 day contract – less expense, and you’re not locked in.
I’m on 3 – will be leaving later this month when my contract comes up. Inflexible customer service (which is a shame after an excellent start) and they upped my bill a couple of months ago. It’s in the Ts & Cs they can do that, based on RPI, if I remember rightly. NO get-out clause. Very rubbish, and I was not impressed. Virgin Media have just done the same with my broadband / T / phone package, and you can turn down their offer an cancel your contract.
Other than that, I’ve never hit the buffers of 3’s unlimited contract and I’ve had zero problems with connectivity or service.