For the record, the villain classes in the Iron Heroes Bestiary alone are definitely worth the price of the book, especially at pdf prices.
They allow you to present a CR-appropriate encounter without worrying whether the PCs can abuse the villain's gear afterwards. They get their treasure as a reward (read: gift) from a noble for defeating the demon (or whatever). So, for example, you can run scenarios in a high-medieval or oriental setting where the PCs would never THINK of looting the bodies of their opponents. It's a win-win. So yes, I'd say the villain classes can work fine in other systems. They're CR-balanced, so no worries about adding magical gear to them.
And I woulda bought Mastering Iron Heroes just for the Villain classes, but Mike's variant take on NPC classes is also good, and one could argue that Zones and the variant XP rules are also worth it all by themselves. Definitely, it's a great value for the price.