Did it on 1st january this year, well worth doing and it should be possible to fly out of KK on day 2.
On day 1 you walk up to the pretty cushy 'mountain lodge' around 2/3rds of the way to the top which was built by the malaysian army using materials flown in by helicopter. You then get an early night and leave at around 2am to get to the summit for sunrise. Take spare headtorch batteries and waterproof gloves - my hands got soaked on the wet fixed ropes near the top and it can be pretty chilly before sunrise.
Once you've got your summit pictures, you head back to the lodge for a second breakfast and then straight back down to the park HQ near the road. We got there by lunchtime on day 2, so it should be possible to get a bus/taxi back to KK for a flight in the late afternoon/evening. Busses can be a bit unreliable (you have to stand on the main road and flag them down) so i'd probably pay for a taxi if on a deadline for a flight. Obviously you'll want to walk at a decent pace, but it should be OK - my GF managed it fine and she's not a very experienced walker.
I wore trail shoes with my mtb cleats still screwed in, you don't need anything particularly heavy duty. Take some warm clothes for the top section and good waterproofs - the weather can be very unpredictable. Oh, and start early on day 1 as the rain normally dumps early-mid afternoon. If you start early then you can be sat warm and cosy inside the mountain lodge by then, sipping a warm drink as you watch legion upon legion of japanese tour groups arrive cold and soaked through...
Good luck and enjoy!
