As per the above two posters, the train is a good way to travel.

I took the second class sleeper SIN - KUL and the first class cabin on the way back, KUL - SIN and I'd use it again. Booked through Ktmb.my and a bargin considering it saves a hotel room.
In first class you get a small, basic meal in the evening and morning, a bed to sleep on (max two per room) and they come with reasonably spacious ensuite showers (when you consider you are on a train).

Only thing I would point out, is if you are using the night train, try to get asleep as soon as possible as you will get woken up at the border in both directions and this often cuts your sleep time a tad short. That said, travelling SIN - KUL, don't bed down until you've passed the border! I found a few co travellers used up all their mosquito spray and unpack their belonging only to find the carriages stood open at the border as all the pax passed through immigration.

I also believe that if you travel in the first class berths there is a first class lounge at either station, but this may not be open on weekends.