Well there is an even cheaper way from airport to HK Island (Tin Hau is last stop), the E11 bus. It is a $21 ride. The advantages of this bus is that it leaves from a stop just several yards from the A11 stop at Airport, one doesn't have to do any transfers, it still has a luggage rack for your stuff, seats are still quite comfortable.
Well the E in the name means it was established for the employees of the airport to use. So that means the ride from airport takes almost 30 minutes longer than the A11 as the E11 bus goes all around the buildings of the airport to pick up airport staff, then before it heads out for the island.
So if I had time, I would take this bus as I like to relax and hate dealing with transfers esp with luggage.
There should 'E' versions of all the 'A' airport buses, so check the bus directory at the airport or check this website.
http://www.nwstbus.com.hk/conten ... page=cityflyerRoute
If I didn't have to lug heavy luggage or wanted to go to places that the buses don't go, then I would use aaa888's method of getting around as the mtr system is very convenient.