Watching what and how they do after a month is as valuable. User interfaces have two sides in this regard: initial experience and boring repetition experience. You can optimize for the former, the latter or both.
Good point. My experience is also that most users don't learn many of the neat features I have built in to speed up their work.
Even if I show them a couple of times, it is often quickly forgotten.
Just seeing the moments of hesitation, how the mouse is moved while searching in menus, etc gives me a lot of information.