I use Lyft and Uber frequently to get to the airport. The estimated amount is always very close to the estimate when I book a driver. I can't say how often it's higher, but a few cents higher based on traffic is expected/acceptable.
Worst experience was when Uber was newish in Denver. Early morning flight with (relatively) long wait time + 2x surge pricing. Driver took about 30 minutes to arrive then canceled when the app showed was 2 blocks away. I then booked another ride at 3.5x surge, and the same original driver accepted. She showed up in less than a minute! Complained to Uber and was refunded the entire fare, but I hope predators like this are quickly removed. Taxis were bad, but this woman was criminal. I hope Karma comes her way.
Regarding Lyft and Uber, head and shoulders above traditional taxis!