Looks: The book art, layout and design is very good! It has some nice different pictures. Really like the "bestiary" art.
Writing: Writing is superb, very clear and intuitive. Really well written.
Content: I have to say that I'm a fan of the One Roll Engine. I'm not going to talk much about the system, you can get a glimpse of the One Roll Engine on the free Nemesis RPG over the web or the Godlike Quick Play here on DriveThruRPG, but Reign makes it even better with some tweaks and additions like rules for Spells and the awesome rules for Companies (which you use to easily manage and control organizations, countries, guilds and the like). The Reign Setting is also pretty good, it's not your default Tolkien/D&D fantasy setting (which for me it's a plus), it has some nice new ideas to it which makes it a nice reading.
Totally worth it! Also, the designer released a lot of small supplements, tools, settings, additional stuff and some very cool things and compiled them into 3 really well done free PDFs over at his website.
Obs: Just as a note, there is another version of this book called Reign Enchiridion (which is also available for purchase here on DriveThruRPG). It's almost the same book, but without the chapters and information regarding the Reign Setting, which is good if you want to just use the system to create your own worlds, and it also includes the contents of the supplements I mentioned.