Defeating dictators is easy. Controlling nations is difficult. Constructing new nations is more difficult. All of this could have been anticipated, and a more accurate assessment of the challenge might have affected the decision to some extent.

Against the cost + interest of the war one has to offset the cost of sustaining the stalemate that preceded it. If the current Iraqi government survives and manages to push oil production up to 6mbpd or so one would have to factor in the impact of additional supply on global oil prices. And of course the world looks better without Saddam Hussein sitting as a head of state, another of those unquantifiable benefits.

I doubt that anyone will ever come up with a full, credible, agenda-free assessment of cost and benefit, or even that it would be possible to do so, given the non-quantifiable nature of many of the factors involved. If the current Iraqi government survives, remains relatively neutral, and resumes oil production the operation will probably be slotted into the "qualified success" category, whether or not it really belongs there.