I've done the Aberdeen, Inverness & Penzance Sleepers (but not since First took over the Scotland ones) and have managed to get some sleep on all of them, I find that something to help you sleep can help a lot (especially if very bumpy).

I'm slightly biased though as I prefer them to flying as getting to the train station is easier than the airport, and I find I save time using the sleepers as the time I'm sleeping is being used for the journey.