The book's premise is that magic exists and can be used by certain gifted individuals (known as 'mancers, with various prefixes) to good ends or bad. What makes this book's universe unique is the centrality of the role of 'flux', or karmic blowback, from the use of magic. Like the crash after a drug high, the flux is inevitable, and experienced users must learn how to plan for it.
What I especially liked about the book was the moral motivation behind protagonist Paul's and villain Anathema's characters. The 'bureaucrat as superhero' idea is novel in itself and FS could have just played it for comedic value and left it there and it would have been fine. But what's truly new is Paul's guiding insight that "bureaucracy is civilization; paperwork is civilization". And conversely, Anathema's psychotic fixation on going back to the primitive world.
I read 'Flex' in a day, and I'm looking forward to reading more books in the series. Highly recommended.