I am both surprised and yet not by the massive negative response on here
Anyway, putting that frothing to one side I must admit my first impressions were very positive. I sadly only got to sample the 1145 from Paddington to Reading (looking forward to getting more distance in the future) but it was excellent.
I was in first but poked around Standard as well but will need to have a proper length journey to make a final judgement. My first impressions of standard are that the green isn't as garish as you'd think in person and that the carriage is very light and airy (though I am concerned that in darkness the lights may seem very bright). The seat backs are at a good compromise height between what the safety requirements and allowing you to see around (remind me of 185s in that regard). The legroom is absolutely exceptional! It was like every airline seat was a priority seat and speaking as someone who normal ends up with their legs wedged against the seat in front this was massively impressive. The seats are firm, there's no way round it, but from the brief experience I had of them they were still quite comfortable but then I'm not one of those that think the greatest seat was the IC70 (the original standard class HST seats that can still be found in EMT or GC HSTs) and everything since then was a mistake

The proof will, of course, be in the sitting so I'll have to withhold judgement until I've done a decent distance on one.
First class I'm very impressed by it's not quite as classy as the HSTs with it's nice GWR logos and colour scheme but the seats are, again, firm but very comfy. The power is nicely placed and the tables are a good size. Bit disappointing that the 5-cars don't have very many tables for two but that been the way since GWR refurbished their HST first class anyway. Again lights were bright but not overwhelming and, again, I'm worried about darkness. But these are all things that could be tweaked or changed as things go on.
In terms of performance the thing went like lightening on AC from Paddington. It was like effortless power I had a friend reading off the speeds from his GPS and we flew through the 80s and 90s into 100s without even feeling like you were under power. Diesel was nice and quiet and not at all intrusive by far the quietest DEMU/DMU I've ever been on. The slope was noticeable between carriages (as was being sat higher which was nice actually) but was certainly not Everest to be scaled.
The ride was very smooth on the whole and the whole thing just felt right and very much like the future unlike the HST back to Paddington which very much felt like the relic of the past that it was.
That being said I do have one concern and that's mostly around the size of the gangways and the vestibules. They did seem very tight and so I have concerns about getting loads of people on and off of them in the rush hour. Though possible it was just that first class vestibule (which had an equipment cabinet in it) and the others, particularly standard class, are larger?
I have no doubts that they will be excellent trains and will serve the public very well even if the enthusiast fraternity find any reason to continue froth about how terrible it all is and if only they'd listen to me!!!!1!!!!!!1!
Personally I now can't wait for them to turn up on the ECML. Much rather have an IET than the Mk4 I'm currently on!