The 'Groundwork' is the seminal statement of Kant's moral theory -a short book, but nevertheless immensely rewarding. Kant is famed for being notoriously hard to read, and this book certainly live up to that impression. However, the introduction by Korsgaard provides the reader with a good impression of Kant's ethics before even having read the text, so that whilst reading it, one can see beyond the moments of philosophical jargon and grasp the theory which Kant is attempting to communicate.
It takes a while to digest Kant's work, but it is a short one, and it is up there with Mill's 'On Utilitarianism' and Aristotle's 'Nichomachean Ethics' as one of the few truly great texts of ethical philosophy.
Kant might be hard to tackle, but things which are testing and difficult are often the most gratifying, and worthwhile. Kant's philosophy is certainly tricky to begin with, but I think the initial difficulty of finding one's feet in Kantian ethical philsosophy is overcome by the magnitude of understanding gained by finding them.