I believe the main reason that this book remains the standard graduate-level E&M text is "inertia". Your prof used it, and so you will, too. As a physics text, it supplies more mathematical details than physical insight. Confusing mathematical expressions with physical understanding may be the reason that many people get the warm and fuzzy feeling about this book. The truth is, most people who proclaim to enjoy this book probably haven't throughly mastered the -physical- contents of Griffith's undergraduate text. If you truely have, you can go a long way without touching Jackson. Sure, long math equations can be orgasmic, but it's not physics.
The explanations in this book for the most part can best be described as turbid. For a particulary hideous example, try the section on the vectorial diffraction theory, and come back and ask yourself if you really know what the heck he's talking about. And then ask if HE knew what he was talking about.