Booked these guys for a transfer from the airport to Hanoi and then for the return leg 10 days later. Booking service was very prompt- there was a small amount of confusion on the return leg but they monitor their emails almost constantly, so it was all sorted very quickly.
The actual taxi was huge and very comfortable- our driver was advertised as speaking no English but he spoke enough to have a conversation about the local sites etc. There was cold water and wifi in the car and the journey was safe and smooth.
Price was exactly as advertised when I booked through their site. Highly recommend.