For those who want to know what the experience is like: switch off the lights, watch full screen, and turn up your stereo. If you have a good speaker system you will feel the full effect of communist Vietnam's night trains!
I would only recommend taking this train if you will spend a LOT of time in Sapa. Otherwise the 2 sleepless nights close together can really drag your vacation down, like it did ours.
Flying is fast and relatively cheap... unfortunately you can't get to Sapa by plane!