It's a good point about the ticketing. However, trains don't get stuck in traffic (generally). The two airports I fly into quite regularly are London Heathrow

It's a good point about the ticketing. However, trains don't get stuck in traffic (generally). The two airports I fly into quite regularly are London Heathrow and Vienna. At both locations I'd prefer to catch the Airport Express train than mess about with taxis in traffic. I was in Hong Kong a few months ago and it was extremely straight forward to catch the airport train. Very, very obvious where to go, how to buy tickets etc. Try catching a cab from Sydney Kingsford Smith airport into the city during peak hour. You will sit on the Eastern Distributor for a long time.