Apart from a few problems with the Booking Form on the webpage (probably my fault and now revised), I have absolutely nothing to complain about.
They are always there on time. Communication is good. There are fully aware of any delays to flights / road traffic. The price is competitive and the transport good quality.
The advantage over a standard cab is that they monitor the times of the flights, so will make adjustments if necessary. Especially useful if the flight is delayed (or even early).