Not only does Butler tell a captivating story, she connects the dots between racism and poverty in the US. She deals with climate change and its effects on a USA under a fascist takeover (and the results in the second book) in a very visceral way.
It is also a deep reflection on the nature of God outside established religions.
If there ever is a hall of fame of sci-fi, this duology belongs there. It is at last on par with Asimov's "Foundation"