The Sigma 180 mm macro lens has been hard to find recently; and this might be the reason why http://www.photographyblog.com/news/..._ex_dg_os_hsm/
A new 2.8 version just launched.
Although no indication of price yet.
Certainly something to consider for the really keen macro shooter.
My F3.5 version has served me well, so hopefully, I won't be purchasing this new model too soon.
One of the big drawbacks with the previous version was a rather slow and erratic auto focus, although many macro photographers prefer to focus manually.
The new entrant is supposed to be much faster and more positive in focus.