All these posts and not a single mention of the wonderfull sigma 180mm macro - its pretty much just as good as the canon whilst also being lighter.
Juza Nature Photography
Juza Nature Photography
For some reason the tamron 180mm has never been a popular 180mm lens on the market - I think because whilst it offers the image quaity its other features are less than those offered by the sigma and the canon (eg the tamron has no HSM/USM focusing).
One downside is taht sigma have recenty discontinued the 180mm macro; might mean they are releasing a new OS capable macro lens soon or that they are letting the very similar featured (and slightly lighter) 150mm take the long range market alone. Certainly there is little difference between the two and the working distance difference between 180mm and 150mm for macro is going to be very slight.
So I'll put a vote in and say consider the sigma 150mm macro lens as an option.