The Eiffel Tower is notorious for its lines. You'll likely wait to go both up and down by elevator. While the elevator is worth whatever wait is necessary for going up, I actually prefer to take the stairs down.
There are three levels. While the stairs are not open to the public between the second level and the top, the stairs between the second and first levels, and between the first and the ground, are wide open. It takes three minutes to bop down the stairs between these levels...as demonstrated in this less-than-smooth video.