I was recently at the Mumbai Domestic terminal (Jet Airways, so maybe 1B) and there was a very long line to get on the transfer bus. I instead got pre-paid taxi (easy, no risk of getting scammed) and went to the ITC Maratha hotel for a couple of drinks (Rs 140) and then got a car to the International Terminal (Rs 150). However, I could have just got the taxi all of the way to the International Terminal without the pitstop. My impression is that this is a lot faster than the free bus... And not very expensive. Just an alternative.