Before starting to use TeXmacs I tried LyX. It is true that the typesetting is not optimal, as often pointed out by TeXmacs users, but for me that was not a dealbreaker. I found LyX to be extremely un-customizable. TeXmacs has Scheme that allows you to do potentially anything! That made me very happy coming from Emacs land.
An important thing for me was having context-aware keyboard shortcuts (that are barely possible in LyX).
Another cool thing that seems to be possible that I plan on exploring next is integrating math typing in TeXmacs with sympy and or Maxima. This would allow symbolic manipulation using the above packages directly in TeXmacs.