I am from Malaysia and have frequently taken the bus ride down to Singapore. It's 5-6 hours in total and is actually a very comfortable ride. They are many luxury buses plying the route and would highly recommend it (if you have the time) since the time taken to get to the airport/pass through security etc will end up being more or less the same (less comfortable) than the luxury bus. (Some of them nowadays have personal touch screen entertainment systems and large reclinable business-class like seats)