So I made it for a quick Reading and back...here's some thoughts (live from Coach K seat 60
).
* Nice and quiet, smooth riding (though gets a bit bumpier at speed, no Mk3 sway).
* The lime green isn't quite as unpleasant as it could be, though it does cheapen it a bit. Same as the newer Southern Electrostars. Grey seats and wood effect ends work quite well.
* Small bog adequate for having a quick #1, though the doorhandle was a bit awkward.
* Someone eating a curry in this coach (why don't other TOCs display the "please don't eat hot food on trains" Merseyrail used to?) - however the aircon is good enough that I can only just smell it 3 rows away (edit: 10 mins later smell gone completely). Temperature spot on, same as a Desiro (so cold for some). Aircon fairly quiet.
* Seats as I thought are the same as the ScotRail 385 1st. I don't mind them being hard (indeed I like ironing boards) but they aren't shaped right for a long back (like Turbostar seats) and so I have to slouch to be comfortable otherwise my shoulders are pushed forwards by the headrest. But...
* ...legroom is excellent, best of any UK train, so this is no *great* issue. Could be better, though. I'd rather ironing boards.
* Flickery LED PIS - cheap components? Could be an issue for epileptics?
* Lighting not bad at all - a warmer colour than the GWR norm, and well diffused. LED based but unlike the PIS not flickery. Not perfect (could be classier), but certainly on a par with other acceptable stock like Desiros.
* Overhead racks massive, would take even the largest bag easily. Who said this wasn't allowed any more when they built terrible designs with little more than a coat rack like the Turbostar and Adelante?
* Tables robust and laptop pull-out handy, though they don't go quite horizontal. There is a bit of a judder making the screen a little hard to read, though.
* Smallish windows, but in the "high floor" coaches positioned just about perfectly for a good view out, as long as you don't get a wall seat.
* Fairly narrow, like 180s I catch the seats walking through being quite broad.
* Acceleration and braking very quick.
* Blinds present, these are a right nuisance because they cause arguments - please get rid in future designs or put up signs making clear the state in the event of dispute like the French do for windows.
* Reservations appear not to be working.
* Step up into "high floor coaches" quite noticeable (as is the ramp at the corridor connection, a bit like SBB EW I/IIs), but as mentioned I find the window height in these preferable. My head is close to the ceiling in those coaches, though. Should have done the whole train like that. Note that the ramp is at the corridor connection, so if you don't walk through you'll only notice the high (as per Voyager or 185) floor.
* Fair bit of vibration at speed, possibly some fine tuning needed.
* Screw covers in seat handles will be gone within a few weeks, too loose. Similarly the folding coat hooks won't last.
* Seat supports positioned such that the window side one doesn't get in the way of your foot. But like Adelantes not bolted to the wall, probably causing the judder noted above. Poor design feature there; I expect they'll develop more wobbling over time. Surprised they were not cantilevered.
* Sloped top of heating conduit so I can't put my foot on it.
Not tried 1st, GWR don't offer a decently priced First Off Peak or I would. Maybe another day. Looked OK though.
So overall...not bad, though not perfect. Take the seats out and replace with Grammer IC3000 (or even E3000 as in the Desiro seat) and I would be sold, though.
Update re seats: 25 mins in and my back really doesn't like the shape. I suspect fine for a shorter person though. Definitely prefer ironing boards.
Edit: turns out we were running on diesel for at least some of the journey. Wasn't audible - I can hear more engine noise in the rear coach of this HST (which, notably, is on one of the other IEP diagrams - so check before you travel!)