Yes, I transit KUL several times a year.
Customs is a breeze. Get your passport stamped, go to the carousel, pick up your luggage, wheel past the guards at the sliding door.
Now, for what is important:when you get off the aircraft, walk quickly to the tram station. Do not sightsee or you will end up behind a huge herd that will make your life miserable as you wait forever at customs.
BTW, when you do get to the tram station, don't stop - walk to the far end. This will also put you ahead of the herd as you exit the tram, walk through duty free and go down the escalators.
Avoid refering to the city as "Kuala Lumpur", as this will mark you instantly as a low-time tourist; call it "K L" ("kay el"). This will save you some grief from those that target hapless tourist types.
All the best, James