I used to own the 180L macro some time ago and found it to be too soft. After owning it for about a year and a half I wound up selling it. I am not sure if I just had a bad copy but I do know others who have had similar issues. I also own the 100L IS macro and the MP-E 65. All three are completely different macro lenses, but in terms of pure sharpness the 180 was easily in last place.
The 180 is a useful lens and I used it quite often when I traveled to Mexico for photographing lizards. It also does a good job for butterflies. For dragonflies I find my 300/4 a bit more useful because they can be a bit skittish. For flowers I tend to prefer the flexibility of the TS-E 90 + extenders.
I absolutely love my 100 IS macro. It is a sharp lens but the main drawing point is the macro. I use this lens quite often when traveling in markets and shops to photograph small things and I almost always do this hand held. Without the IS it simply wouldn't be possible.
If Canon were to come out with a new version of the lens I would probably pick it up once reviews confirm that it fixes the sharpness problems of the original.
You are the first person I have heard saying the 180mm is soft, if anything this and the 135 f2 are perhaps Canon's sharpest lenses. Maybe you had a misaligned copy and thats just unfortunate.
You are right, in the macro world, the smallest things are amplified, so this and the 100L are quite different. Also I owned that one too, it was sharp, less sharp than the non;L version but very close, but the 180 seems sharper.