Anyone interested in ethics (moral philosophy) must read this work. Of the handful of indispensable moral philosophical works, along with Plato's Republic, Aristotle's Nicomachean Ethics, Aquinas' Summa, and Mill's Utilitarianism. Relatively short (but dense; he is German after all), the beginner of Kant's philosophy should start here, and then advance to his (arguably even more influential) epistemology. In my opinion, it's easier this way than to tackle the 1st Critique first. As profound as Groundwork is, this course of study eases the novice into Kant's philosophical system. This is the way I would teach Kant.

The fundamental moral challenge of the Groundwork: there is no necessary correlation between goodness and happiness (an oversight by all other moral philosophers and religious thinkers, according to Kant). As such, the moral person must choose to be good rather than happy. Kant is unrelenting and unforgiving of moralities which too easily equate the good life with the happy life. So powerful is his argument that this may be a dangerous book to read, as it may alter your life.