With respect to all the posters, there hasn't yet been a single post which actually compares the two from anyone who has had both.
I initially had OME on my Defender and it was great until I started carrying weight in the back. Couldn't get the set up right at all after that, it was too stiff and bouncy, handled poorly on the road and the rebound from the shocks was just too much. In the end changed to Iron Man B springs and nitro shocks and it was SO much better. Better road handling but still a sharp turn in, less bouncy, yet still compliant offroad, and no rear sag. This was with front winch, big bumper etc. The vehicle sat level.
My 80 has Iron Man Nitro shocks and C springs and I can't find fault with them at all. I find the biggest difference to handling and feel is caused by playing with the tyre pressures. At 35 psi all round yes it's great for carrying weight but it's just too bumpy, I feel every pebble. At 30 psi all round it's brilliant. Just the right about of rebound from the shocks, stiff but not bumpy, handles well on tarmac.
But I would say before you start saying this one is poor, this one is great etc, just have a look at the weight you're carrying, your tyre pressures, your driving style and the terrain you're using it on. As far as I'm concerned, the biggest benefit with Iron Man is that it's so much cheaper than OME, for a product which is pretty much the same yet you can play with the spring stiffness by choosing different types that the firm offers and they're matched to the shocks. I guess everyone on here knows that West Coast is the sole importer of Iron Man, so I would say talk to Paul about your intended use before you start buying OME.
And to the guy who has an Iron Man kit and is "not impressed" with it, you don't say why, but again, I'd say talk to Paul before you whinge on the internet, I think you'll find him pretty helpful.