Frstly, Foucault *is* difficult - there's no getting around the fact that he's frequently, and not unintentionally, opaque, complex and, sometimes, just deliberately tricksy. Rabinow writes a coherent introduction here to place Foucault into context, and reviews all his works, drawing together pertinent themes.
The book itself offers selected extracts from the longer works, and full essays in some cases such as 'What is an author?'. My criticism is that it would have been helpful to have had brief introductions to each selection/essay as well, putting it into specific context.
Whether you choose to see Foucault as a post-modernist or a post-structuralist, or accept his own negation of any kind of classifying 'grand narrative', it's practically impossible to be a student or scholar today and not engage with his thinking.