I've run a game since 2007 that started as Iron Heroes, converted to 4E, and now converted back to Iron Heroes + Pathfinder.
Here's what I've learned:
* Iron Heroes is an awesome ruleset for a Conan or Celtic style campaign.
* You have to restrict the players to IH classes.
* Don't use Arcanist or Spiritualist. PCs in IH should not have magic.
* Use monsters, adventures, and additional feats from Pathfinder.
* Creatures that can only be hit by magic are much more deadly in IH.
* To keep the feel, have at least 60% of enemies be mundane (i.e. orcs, humans) and save magic for really creepy or evil foes.
This has worked well in my campaign. We have 4 PCs played by grown-ups (age 30+) who are looking for a gritty, challenging, Conan-style world, not a Forgotten Realms where magic swords and potions are available at the general store.
Happy to answer further questions!