Direct vs Iterative Linear Solvers for non-linears

bill

I would like to hear users views on the observed differences when using direct solvers vs iterative linear solvers for highly non-linear problems in either structural or fluid dynamic problems. The more non-linear the better!!!

I am fully aware of the well known academic differences of speed, memory, robustness and accuracy etc. But I would like to hear peoples real world experience on real world complex problems. Have there been any problems where iterative solvers were just not suitable or has a solution been ground out eventually?