Only for special occasions & job interviews.
I work with a lot of seriously nasty chemicals that occasionally require a respirator, so I can't wear any at work. Not that I would want to--in other cultures, makeup is thought of as kind of tarty, and I work in an extremely multicultural department. Also, I prefer my beauty rest to getting up early to do the war paint.
The field I work in has a kind of uniform: female scientists of a certain age and responsibility level are supposed to be makeup-free with short hair and wear khakis, Dansko clogs, plain blouses, ugly sweaters and glasses. Grey hair is a plus. If you dress more feminine than that, people think you're not very serious about science. Don't ask me why, that's just the way it seems to work. The girly chicks either morph into the uniform or they don't get promoted.