They did a good job of fusing the D20 Modern rules with Pathfinder. The book is well laid out with clear chapter separations similar to the Pathfinder main book. They did not just shoehorn the modern rules into Pathfinder rules. They use Pathfinder as the core system and d20 modern is very nicely laid over it as a shell. The end product is something that feels similar to d20 modern but has all the developmental advantages of the Pathfinder system!