Had a go on 700114 today, twice in fact - once deliberately and again by pure coincidence that it was operating the service I needed. In true GTR form, both services were around 20 minutes late.
Thoughts -
The sound level and general feeling of refinement is excellent, on par with Desiro UK stock. Sitting in a trailer was very quiet indeed, at least to the standard of an Electrostar trailer which given the open gangways is quite impressive at speed. Sitting directly above a motor bogie for my second trip, the noise level is very reasonable, possibly not as quiet as a Desiro UK in that regard, but very reasonable and much quieter than an Electrostar. Acceleration was in parts fairly brisk even on DC at low speeds and smooth throughout, ride quality was smooth but damn, I'd forgotten how bad the track is on the Brighton mainline!
The A/C clearly works well, it was arguably a little too aggressive on my train, it did get a bit chilly at times!
Seats firm as anticipated, but they are somehow different to those on late Electrostars. I can't quite pinpoint what the difference is, but they don't feel like the same seat at all. Both are firm but the Electrostar seats feel like a soft coating on a hard cushion, the 700 seats feel more like a hard coating on a softer cushion. The intrusion into the foot space was not as much of an issue as anticipated, it's less difficult but still very uncomfortable to lean your elbow on the windowsill, whereas this is arguably a lot easier on 377s and 387s.
Neither 30 minutes on a 700 on two occasions nor 60 minutes on a 387/1 caused me any discomfort in my bottom or back, and I do suffer from back pains, so I think the complaints about the seating are a little over the top. Yes the soft seats on, e.g. 377/1s are a lot more comfortable, but the 'ironing board' seating on modern units does not seem to bother me as much as is regularly suggested that it would.
Ambience in the unit is clinical but clean (at least for now), nothing realy looks out of place / cluttered and with the trains being as quiet as they were today, they feel extremely spacious. The perks of travelling on a 12-car unit off-peak that's only running half the route I suppose.
The 'new train smell' is not exactly unpleasant but quite strong nonetheless - I wonder how long that will last

Door chimes are possibly a bit too loud for my liking given how many of them there are, but the buttons are instantly responsive without needing to press excessively hard as per 377/6 etc. and to my surprise, despite pocket doors being used instead of sliding doors, they deal with trains passing at speed far better than Electrostars with plug doors. Full marks here, as long as they prove reliable long-term this is an example of a door system that completely negates the need for plug doors and operates at double the speed.
PIS - arguably the weakest part of the system, its design is fairly well thought out, the audio quality is excellent, the text is quite clear to read as it's very high resolution, the scrolling rate is correct, the LUL information is useful when relevant, the loading indicators and toilet availability indicators seemed to work well enough, however there are some clear issues with this system, as one might expect for the first of its kind:
1. the system was rebooted 3 times prior to the train leaving London Bridge - not sure if this is normal behaviour, but it's a bit messy, you're reading the info then the screens go blank
2. the displays are unstable - on both legs of my journey in different carriages, some screens intermittently went blank when others did not (only the right portion in some cases, both in others), and one spent a minute or so cycling through each screen several times a second making it impossible to read any of it. The software for these is clearly still quite buggy and while fortunately each screen seems to operate independently of each other (as you can observe them all changing at different times), not sure why that should be the case and I suspect it will lead to a 'de rigeur' of at least two or three displays not behaving properly on each 700 in perpetuity unless they can improve the stability of them.
3. the LUL information needs not be displayed once outside the London area when travelling away from London in my opinion.
4. when the 'the next station is' message comes up, surely the second line could be used to continue to reel off the next stations, rather than you having to wait until the next station to be reminded of the calling pattern?
5. The loading and toilet indicators were only used after the train had departed the terminus. While that may be less an issue when the terminus is Bedford or Brighton, for services starting at busier stations I would argue that should be ready a lot sooner - giving people the opportunity to relocate before arriving at further stations that exacerbate the situation.
The general 'feel' of the train is unquestionably a metro service. The 377/6 I returned on felt far more cosy by comparison and the 377/1 earlier in the day positively luxurious, but that is part of the point. The time it took to disembark from the 700 would have been a fraction of what it was on the 377s with me seated behind my full-size table. I totally get why they've laid them out this way and I agree with it. On a quiet off-peak service the sheer size of the train was enough to lead to ample seating available despite the sparse layout, yet there is vast amounts of standing space ready for the peak time crush.
It's an odd conflict to be on a metro-style train speeding along quietly and smoothly at 80+ mph with hardly anyone else around, but in general I think they've done a pretty good job with the specification here (the WiFi and power socket grumbles excluding). The train has a professional finish and appears well-suited to the job. It will be unfortunate to lose the cosy style of the 387s during the hour long slog negotiating the diversion via Crystal Palace, but I won't be complaining too much if a 700 turns up in place of one. I honestly look forward to travelling on one again.