You don't hear much about this lens. I bought one today. I have the Canon 60 and 100 macros, but I wanted this lens for one reason only - butterflies. I went to the local butterfly exhibit several times this summer, and found the 60 got too close, often disturbing the butterflies. And the 300L with 1.4x needed too much room in a very cramped setting.
Then, on my 3rd or 4th trip there, I saw another guy shooting, and he looked like he knew what he was doing. He had those knee pads you slip one, a 5D, a big lens and a macro ring lite. As I approached him, I recognized the Tamron design of his lens. It was the 180 Macro. I asked him if he would be posting his images on the internet (in hopes of getting a link so I could see the results), but he was rather vague about it, so I didn't press it. Anyway, I've been thinking of getting this lens ever since, and finally pulled the trigger this week.
No butterflies around, but thought I'd share some images of the lens, since many here are probably not familiar with it. It got good reviews at photozone except for the micro-motor AF and no limit switch. Over at fredmiranda, many users complained of AF noise/hunting. For my first shots today, I used manual, set the distance window close to 1:1, moved in with my 40D till the focus hit, then set up my tripod.
Based on the size of today's subject, and dried sunflower, I think it may work out OK for butterflies - need to get over there soon before the exhibit closes for the year.
The build on this lens is very nice, IMO. There seems to be some conflict on the weight of this lens. I get 1020g on my postal scale with the tripod ring. The lens has a nice finish, and the case is about on a level of the case for my 300 4L, though not as good-looking.
A couple quick shots. I didn't even see the fine hairs in the viewfinder (my vision's not so good). I will need some practice, no doubt.