I'll chime in with some pros/cons of the Tamron 180mm f3.5. As you probably know this is my primary macro lens.
Advantages:
- Bokeh is to die for....so creamy it's almost sickening.
- Sharp, sharp, sharp. (probably same w/ Siggy 180)
- FL is great (same w/ Siggy 180)
- MF is incredible on this lens. It takes a decent bit of turning to make a focus change which makes fine tuning your focus a breeze and I rarely have missed focus shots at 1:1 (different story when using Tubes and TC's)
- It is pretty lightweight for it's size.
- Works incredibly with the teleconverters, full set of tubes, or combination of both. Even mounted it to my bellows and it works great.
Disadvantages:
Build quality isn't awesome, but it's not horrible. It defeintely doesn't feel as solid as say my Sigma 50mm f1.4 or some of the better Nikon glass, but it doesn't really feel cheap either (like my Sigma 70-300 APO did).
...and the biggest disadvantage...AF is basically useless. When I say slow, you might not understand how I'm using that word. I'm talking if a 18-55 kit lens is a Mustang GT, then a Tamron 180 f3.5 is a Kia Sephia. I thought I would be able to double it w/ TC as a bird lens, but it's just too slow.
That said, it works great in MF for zoo trips. I have yet to take it to the zoo w/ the 1.4tc but I'm sure that it would work great and it is super sharp for regular animal shots...just realize you'll probably end up using manual focus. I can post sample shots of non-macro animal shots taken with the lens if you'd like.
All in all, it is an incredible lens if you are aware and okay with it's limitations. I've had contemplations of selling it for a Sigma 150mm f2.8 which has very fast AF (from what I've heard), but don't see the point since the Sigma 50-500 OS is on my list and would trump the Siggy 150 for animal photos.