So, an update after half a day...
It seems to work pretty well, with the usual caveats. Names. accents, etc. There is even live correction, which is really weird, given I read that the donkey work is being done over the internet.
Problems include the setup shows the subtitles over the presentation, so if a presenter has used loads of small text... and in most of the seminar rooms, the presenter can walk in from of the screen, thus rendering the subtitles illegible...
We have even had "Thought Leaders" in the subtitles in one session. Sadly I wasn't quick enough to grab a shot of the screen when that came up.