I've used the Caledonian Sleeper a couple of times. The first time it worked reasonably well, I got a reasonable amount of sleep. The second time not so much, not really any fault of the train, I just didn't relax enough to get a reasonable amount of sleep.
Both times I'd say well rested would be a stretch. The first time felt a bit like a magic carpet in that I got somewhere while using up no useful hours but it still wasn't perfect.