Is ambition a virtue?

Is ambition properly regarded as a virtue? Ayn Rand defined ambition as "the systematic pursuit of achievement and of constant improvement in respect to one’s goal." If we apply ambition only to rational goals -- as happens with the virtue of integrity, where loyalty to values only constitutes integrity if those values are rational -- then could ambition be considered a virtue? Or at least, could ambition be an aspect of a virtue like productiveness?