Sometimes when I'm at HKG (https://www.yelp.com/biz/0ymPFKNXPC1o2fc9XtyuVA) I end up people watching tourists as they rush to the Airport Express (https://www.yelp.com/biz/Pl9LmJKIOnCQtXIWEDAScQ) or speed away in a HK taxi, and wonder to myself if they would ride away like me in the most relaxing ride back to the island in the form of the airport buses.
First of all, it's the local's choice. (#1) More people line up for this A21 because it hits many if not most of the popular hoods in Kowloon. It is also only $33HKD each way, payable via the Octopus card. Drivers aren't likely to speak much English, so have it ready upon entrance.
There are cameras all throughout the bus. If you think your luggage is going to get stolen, you can always keep your eye on the monitors within the bus. There is also semi-decent WiFi on the bus and most everyone will be on their smartphones within minutes of hitting the road.
The ride is smooth and easy and like all things in Hong Kong, the AC is on fleek. I personally like to zone out with these rides, which is a nice departure from the fast paced nature of the city's culture.
Of course, this ride isn't for everyone. It's a good 45 minutes pending traffic and sometimes you might end up sharing your seat with some random traveler who may or may not fall asleep on you. (#2) Hence, avoid the peak hours and you should be able to covet your space without much interruption.
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(#1) i.e. the most affordable option. This is not a stereotype.
(#2) If I have somehow done this to you, my apologies. read more