I've got one in my group and, man... it's like he's playing a different game from everyone else. A truly epic, him-against-the-hordes one, which is sort of awesome, but it's not really what I come to WFRP for.
I think they're a little OP. Starting out with plate armour (2 def 5 soak), plus the 4-5 Toughness any self respecting dwarf warrior will start with, means most damage bounces off him, meaning minimum wounds, also meaning crits aren't likely to affect him either.
If I were going to start over again... I think I'd houserule Ironbreakers as an Advanced career. In the meantime, I'm not sure how to put together a challenging fight for this party that will challenge the Ironbreaker without overwhelming the other guys (not that they're wimps either). Anybody have an idea on how to put together something reasonable that can give these guys a run for their money?