I loved the Sigma 105mm, which I used with a Nikon D70s. Many pictures in my gallery are taken with that lens (dont let that deter you though!!). I found the lens extremely sharp and it was amazingly durable. I used it in tough conditions of extermely high humidity and dust for extended periods of time. However, the AF was quite slow, but I suspect that makes very little difference for 1:1 macro pictures, where I found that manual focus was easier anyway.
Hope this was useful.