I stayed at the JW Marriott in Mumbai and had a very pleasant stay. The Exec lounge had a a superb breakfast.

I was in Mumbai in transit and had 2-3 hours to see what I could of Mumbai. If you are in a similar situation and you don't mind non-luxurious transport then the best way to go downtown is by commuter train. Despite the Marriott staff warning me several times not to take the trains, I did so. I took a cab to the local station. The queue for tickets is extremely long but negligible for a first class ticket (which is still probably less than a pound or so). The roads are heavily congested and taking a taxi all the way will take considerable time. The trains are crowded but no more so than a London Underground train during rush hour.