I went from Sin to KL about 10 days ago. Went to the train station to find out that there were no tickets available.
Then went to Golden Mile Komplex on Beach road. There are many travel agents. I shopped around a little bit and was quoted anything between 35 and 59 S$.
Settled for the 35$ one since I couldn't really understand the pricing difference.
The bus was the most comfortable one I have ever been on. An 18 seater with fully reclinable seats. It took little over 6 hours all in all. (Started in SIN at 1 pm and arrived in KL about 7 pm)
I had a (rich) friend with me who, on it's own would never ever consider anything but a flight, and he said he enjoyed it.
Highly recommendable.