Much has been said about the Mahabharata by people much smarter and much more familiar with it than I. It's an enduring tale, a prodigious tome, and one of the foundational texts of one of the largest religions and one of the largest nations in the world. It's obviously a powerful story.
On the recommendation of a friend, I spent much of my reading time this year on this bad boy. It's brought me to a deeper understanding of some of the assumptions of Indian culture, and it's given me more than just the embarassingly basic knowledge of Hinduism that most Westerners get.
Having done so, I'd hardly qualify it as a must read, any more than the Bible, the Iliad, or Beowulf are must-reads. However, it is one of the easier portals to learning more about Indian myth, and definitely serves a purpose if you want to come to greater understanding of world culture.