I had my first trip on one today albeit a short trip from St Pancras to Blackfriars.
Overall, better than I feared. The general feel was bright and spacious, and ride quality was excellent.
Compared with my usual diet of 450s the leg room seemed fine and the aircon was much better, not the tropical stuffiness that I'm used to. The PIS was working fine, but no loadings info, and the doors released after about 2s at all stations. The flooring was the same textures finish as on the 458/5s, which SWT seem incapable of keeping clean.
On the negative side, although I found the posture of the seats fine, they were like concrete, not helped by the astroturf covers, and the lack of spacers between the seats and between the window seat and the side is an issue. As already mentioned, the heater ducting is intrusive, more so than on a 450, and the cantilever seating prevented me from stretching out, although it wasn't a big nuisance.
So, overall a fine train but better seats would have made all the difference.