I have Nikon DX bodies exclusively and shoot a lot of macro, so I bought both the 85mm f/3.5 and the 40mm f/2.8 DX Micro-Nikkors to supplement a couple of older macros I have.
I have a weakness for bright lenses and always had great experiences with Sigma. So when I wanted a bright 50mm, I found it natural to look closer at Sigma's offering.
I have always loved bright lenses. If money was no issue, all my lenses would have f/1.4 and f/1.8 stamped on them. A few slow ones would be f/2.8, but only the slow ones...