I've yet to do a full overnighter in the Mark 5s, but on Saturday morning I travelled in the seated car from Edinburgh to Fort William. I'd never normally consider doing a full night in the seats, so it's about the only time I'd ever use them.
Must say I was quite impressed with the seated car, which I reckon is considerably better than the Mark 2s. The seats are spacious and comfortable, and I thought décor and lighting were good. Only niggle is that the coach was fresh out the depot on a changeover, and it was very cold when we boarded - it took some time to warm up, but the TM sorted us out with free hot drinks from the lounge car!
Ride, like a lot of new vehicles, is a touch on the firm side, and was excessively bouncy on the dodgier bits of the WHL.