I've used it, but mainly for geting to London, rather than to the hills - an advantage of living just south of the Highland Boundary Fault, I guess.
It does look a bit tired, so a refit would be nice (and give us toiletries and breakfast again - they didn't in April, when I travelled without a toothbrush expecting to get one on board).
As to the time it takes - sleepers everywhere take ages, so that you (1) get enough time to have a reasonable sleep and (2) don't arrive at the destination at stupid o'clock - for instance the Edinburgh-London service leaves at 10pm so could, feasibly, arrive before 3am, but what's the point of that? It's better to arrive at 7am/8am, get breakfast and be wherever you need to be in London at a civilised hour. It's a sleeper; it's a relaxed way of travelling; there's sofas and a bar on board, and the bargain berths can be had for much less than £100 each way.