Just as a data point, I used to take notes in class with this software (essentially blackboard after blackboard of math expressions). It is possible to type extremely fast with it and thanks to copy/paste/modify you can actually go faster than the lecturer. The wysiwym display is also critical to check for transcription mistakes in real time.
I was also using the software to do my own derivations. My handwriting is pretty bad so I actually preferred that to using a physical notebook.
Finally I did the typesetting of my whole PhD thesis with it. I had to split it in multiple files because at the type it would be very slow on very long and complex document but apart from that the result looked very nice.
As an aside, and this is just personal opinion, I always felt the love of latex in the scientific community is some sort of Stockholm syndrome. There is no arguing about the fact that latex is better than word for any serious writing and that the output is normally beautiful (if one chooses the right style, probably CV are the common exception) but the input method (let alone the conglomerate of tools/libraries around it) can be greatly improved. If latex notation is so good why then nobody uses it to write math on a blackboard? :) Texmacs (and lyx ) really bring something new to the table. What I think still remains to be solved is the topping to create new templates, right now it is a bit like working with raw css/html. I’m pretty sure something more intuitive could be built, something like the developer tools of chrome would already help.