The estimated fare is very rarely wrong. I have only paid significantly more once and I use it all the timeUsed Uber for the first time on Saturday night in Newcastle, really impressed. 4 minute wait from pushing the button at 11.30 pm knocks spots off any conventional taxi firm, and the fare was £3-4 cheaper than I'd expect. Actually £15 cheaper as I got that in credit when I signed up. Decent car, driver who didn't want to talk shite and no hassle with cash/tips.
When I was playing around with the app early in the evening, around 7.30, there was a 'price surge' to 1.9 times the normal fare, theoretically to get more drivers out to meet demand, but you could opt to wait until they sent a message saying it was back to normal. Absolutely no complaints at all though I have heard of problems from friends in London when they have accepted an estimated fare but been billed for 2-3 times that much after the driver hit traffic congestion, deliberately or otherwise (the estimated fares assume everything is tickety boo).