I mostly agree with Lester's answer to your question in that thread.
However, I haven't "mastered" the 180mm lens yet, actually, far from it 
In the meantime here's a couple of examples:
1- Canon 20d, 100mm macro + 20mm Kenko + MT-24EX flash, handheld: FILESIZE WARNING
http://home.comcast.net/~eldoronki/green_fly.JPG
2- Canon 1DsMkII, 180mm + 36mm Kenko + MT-24EX flash, handheld: EVEN BIGGER FILESIZE WARNING!
http://home.att.net/~hatem/moth.JPG
I'm sure now it's even harder to decide. In my opinion, the 180mm lens is only good for two things: image quaity and working distance. Otherwise, the 100mm lens is a better choice. You can autofocus on an insect easily with the 100mm lens, if that's important to you, with the 100mm lens. With the 180mm lens, it is a different story: first you have to manually focus to the closest point that you will actually use autofocus on. When you're there, the insect probably won't be
)
A tripod is mandatory. The lens is heavy already, and after five minutes of handholding, you'll get the point.