So, as of last night I've now done two journeys on the mk5s. Euston to Inverness and back.
I should declare an interest in that I have a certain sentimental attachment to the mk3s, having done so many journeys in them over the years, but here's my attempt at some objective comments, which can only cover the sleeping cars themselves, as the lounge car and seats remain out of use. And we were in a "classic" cabin without ensuite.
The overall impression I guess, is that they are OK (at least, if you're in a cabin with no major malfunctions, which I was in both directions). Reading some reviews, you might think they are an outright disaster. That would be overstating things. There are some ways in which they improve on the mk3s, but I wouldn't say they are massive step up, on the whole.
My feeling is that the design and construction quality, at a detail level, is quite poor. There are a lot of things that are just a bit flimsy, or don't seem to have been really thoroughly thought through. In this regard, compared to the mk3 sleepers, I think they compare very poorly. Even in the rickety state they were allowed to reach at the end of their lives, the design of the mk3 cabins seemed robust and carefully thought out - as if they'd gone through a good few prototypes, tested them, and then built things solidly and from scratch, to do the job intended.
The mk5s have a bit more of a feel of having being largely assembled from a collection of off-the-shelf components that aren't necessarily exactly right for the job (and which don't look like they will survive many years of heavy use). I know some of this is deliberate. The sink and tap, say, look much like a sink and tap you might find in a small hotel room rather than on a sleeper train. That's part of their thing about making it a "hotel on wheels" but it doesn't result in the kind of clever compact design that you see when something like this has been done really well.
The space to stow luggage under the bottom bunk is pretty badly designed. Fair enough - there's only so much space available, and it can only be a certain height. So we could just about manage to push two large rucksacks under there - but not without them getting all caught up on the flimsy clips that secure the bottom of the ladder. These clips look like they were bought for 19p each from Screwfix. It's not just that they are so obviously going to get damaged though, they are fixed so they kind of fold down (why?) in such a way that they tangle and jam as much as possible with anything you try and push under them.
At the end of the luggage stowage bit there's a sign telling you not to obstruct that area (I think it's the inlet/outlet for the AC), and if you look inside there are some spikes coming down from the top, which I suppose are to try and stop you ignoring the sign and pushing something in there, but they just create another invisible obstruction for things to get caught on.
A friend who used the new carriages before I did told me she thought the ladder was in the most inconvenient position possible. I think she might be right. The mk3s had a much better arrangement for the ladder, I think (will have to look at some old photos to remind myself how it worked).
Another example of a bit of bad design is the table that slides out from under the sink. When you want to put it away, you fold the end flap over, and try and push it back in, and it immediately jams against the little cover flap that conceals it when it's stowed away. So you have to stop and lift that flap up with your other hand. I don't see how this can be excused - this is just rubbish design! I bet those cantilevering tables are gong to constantly have heavy bags put on them, and get broken, by the way.
Aside from these functional things, I don't mind the aesthetic of the overall interior design too much - in terms of colours and so on, if you can ignore the slightly cheap look to everything if you examine it in detail. I wonder how long it's going to last before it looks rather tatty.
On the way back, the door to the shared toilet was jammed. You could just about shove it open and closed. Close inspection of the piston rod that attaches its midpoint to the wall showed that the fixings into the wall were pulling out and it had got bent. Badly specified fixings - just another indication of general poor quality. The other end of that rod is fixed to the door with a joint where the securing nut was loose enough that I could turn it with my fingers.
I mentioned in a previous post about a bad smell coming from the sink. That seemed worse on the northbound journey than the southbound one, so perhaps is an intermittent problem.
The illuminated surrounds to the lightswitches that several others have mentioned are indeed too bright and a disturbance once all other lights are off.
I don't know if the general ride is better or worse than the mk3s. I couldn't confidently say. We were in a cabin towards the centre of the coach (7) so there may be more noise and lumpiness if you're closer over a bogie. There seemed to be more noise and bumpiness from track joints in the carriage we cam back down on, than there was on the way up.
On the way north, I didn't even notice or wake up while we were split apart at edinburgh - neither did my travelling partner. There was however quite a big jolt during the joining process on the southbound trip, which was a little disappointing, as the new couplings were supposedly going to make this a more gentle procedure.
The beds themselves seemed perfectly comfortable, maybe even a bit more comfortable than the mk3 ones.
I can see that the general impression of the common areas, like the vestibules, must be much better to a "regular" passenger than they were before. I'm still bitter at the loss of opening windows, but the clanking, creaking and slightly drafty bits between the mk3s have now been replaced with something more refined, and the info screens are useful (when they are showing info - at one point they'd all stopped showing the carriage numbers meaning you had no idea which carriage you were in).
There seems to be exceptionally fierce aircon in the passageways - sounds a bit like being in a wind tunnel - maybe they have it turned up to max for Covid reasons.
I'll look forward to doing a trip (hopefully) once we can use the lounge car again. For me, that's quite an essential part of the experience, if you're on one of the Highland trains.