I've yet to go through one of these, but I've at least imagined it would be a decent format.

(For me, at least. I don't parallel-task very well in the first place, and being under the magnifying glass makes it significantly worse. I have trouble, say, even making sense of the words someone else says when I'm already focused on coding/reading/writing something. If the interviewer interjects a thought or question about what I'm doing, I often have to stop, struggle to ~park my train of thought, and then ask them to repeat what they said. If I make the mistake of asking them to repeat before the train's parked, I may have to ask them to repeat it again.)

I think I'd also look pretty favorably on an interviewing process that gives a few format choices and lets me choose what I'm most comfortable with.