I will pretty much always take the stairs if it's one or two flights up; after that it's a bit more variable and for five or more floors almost always the lift. This is mostly because I'm too impatient to stand around waiting for the lift, even if it's overall faster to use it. Getting a little bit more exercise is a secondary benefit. I usually take escalators rather than stairs, unless the approach to the escalator is so crowded with people that the stairs look quicker.

Like another commenter I tend to seek out the usually hidden stairs in hotels; in that case there's also a vague sense that they're worth scouting out in case of fire.