Just to provide a couple of data points for those who use the Airport Express, but wonder what the time/cost tradeoffs are for some of the alternatives.

Going to the Jordan area of Kowloon, just north of TST, I took the S1 bus to Tung Chung MTR, then transferred at Lai King. Just 2 stops to Lai King. 8 more stops to Jordan.

Journey time, including waiting and getting to the correct bus stop, and finding the right MTR exit, was about 65 minutes. Cost: HK$3.5 for S1; HK$15.1 for MTR using Octopus. Total: HK$18.6. The S1 was fine, uncrowded and with room for bags. Tung Chung MTR to Lai Ling was fine. Lai King to Jordan was crowded and somewhat inconvenient, even with my minimal convertible backpack luggage. This was about 11AM. Rush hour would be pretty bad.

To get to the S1, turn right, down the ramp, to the bus station area. The S1 is on the far right island, near the car park, in the back. Octopus or cash accepted.

From Jordan area back to the airport, I took the A21 bus. About 45 minutes, including waiting, at around 7:45AM. Cost: HK$33. Room for bags on the bus. Octopus or cash accepted.

For comparison, a round trip on the Airport Express is listed as HK$160, not counting any sort of promotional fare. In town check in on the return is possible. One must get from the in town station to the final destination, which for me would have meant a taxi. Not all that expensive, to be sure.

Verdict: based on this limited trial, the S1/MTR seems ok. I'd probably try it again. It's not really convenient with roller/larger luggage (or immigrant bags or boxes of fragile items--you get the picture.)

(Apologies if this replicates some other thread. Search doesn't seem to be working for me.)