"...let's just focus on the initial invasion itself and whether you would like to do this all over again, vs something like commit more forces to Afganistan and find the terrorists responsisble for 911?"

The alternative you propose isn't really a thought out alternative. For instance are you suggesting we invade Pakistan and/or "follow the enemy" into Pakistan. And btw does this also include going into Iran. If we *were* to do this, it seems it should be covert operation and if you and I know about them, then it's not really a covert operation. And considering that this govt has a pretty hard time keeping anything covert, perhaps a better idea is get some other govt doing this kind of thing.
And generally this whole idea is related to putting more US forces into Afghanistan. Fighting any war, is largely a matter to do with logistics- put simply, Afghanistan is about the worse place in the world in regards to logistics. Because it is so difficult as far as logistics you will have a higher cost per soldier in the field, so to get a force equal to the fighting force in Iraq the total cost could be roughly 50% or 100% more than the cost of the operation in Iraq. And in general terms, a general rule in warfare is not allowing the enemy to pick the terrian or circumstances of the battle [therefore, one of the reasons why offensive is more effective way conduct war, rather than defensive].

Getting to the important point: would you support a war in Afghanistan that was costing 100+ billion dollars per year? And how many years would you support this effort?

I would say the alternative is bogus, but your main question is on whether the invasion of Iraq was a good idea. First of all, this was something that only the President of the United States could decide. I believe generally, Americans should support a President when he/she has to make these kinds of decisions. Second, one has to consider the circumstances of the situation- had Saddam desired to aviod the potential of having the World's superpower decide to remove him from power, he could taken actions that aviod it, specifically, by following the cease fire agreement. And/or if Saddam had simply admitted that invading Kuwait was a wrong thing to do and that rather attempting to delude people, admit that he lost the war and therefore should accept the situation and try to improve his relation with Kuwait and rest of the world. But as we know, Saddam is dickhead and seeing him hang should not cause anyone to feel any sorrow.

Third, the invasion was probably to best miltary operation ever performed. The US miltary did an outstanding job. The State dept did not do as well, but pretty good. Not as good as the US miltary, but as comparison other State Depts [either previous US state depts of other eras or something similar in other countries; they did ok]. The UN screwed up pretty badly. And France was a hopeless case.
Fourth, the present situation in Iraq is not as good as one might hope, but it seems the Iraqi goverment is doing pretty good as far as taking actions to address the various problems, and it's not as though the terrrorists are gaining much support- rather they are more like a disease that is hard to get rid of. And what is fundementally important is that problems are being addressed and you have a democratic process that's addressing some of these problems- which should be remembered, were somewhat hidden to large extent for most of the time. And if Iraq were calm and serene, it would *almost* indicate some kind of mental illness. But we are far from this at the moment, and things are seeming pretty dark for the people in Iraq but saying that we plan to leave them to an uncertain fate at this point in time, isn't making the situation any better.