I'm with O2, and due an upgrade, so looked at the iphone 6 (I have a 5S). I was all set to get one, but O2 has decided to charge a monthly rental for the handset, plus the usual monthly fee. It still works out at around the same price per month that I was paying before I upgraded, but I can't get my head around paying £20 a month for a handset.
So for the time being, I've stuck with my 5S, and am saving £20 a month at the same time.