I personally don't use MS Test. It's a niche product built by a team that was largely detached from the developer testing community. It's mostly visual glitz aimed at people who don't really have the experience to be discerning, and who can be manipulated into buying tools by blinking lights and shiny surfaces. The VSTS product line segmentation is out of touch with the reality of the cross-functional roles that developers are increasingly called to play. It's way over-priced.
There would be little value in me writing extensions for MS Test. The limited community of folks who have adopted it are largely following Microsoft's guidance on developer testing and thus are missing the BDD point as widely as Microsoft missed the TDD point. The open source world is innovating in this space much faster than Microsoft can hope to - both in tooling and the appraches driving the tooling.
The above summarizes my opinions on MS Test much better than I could.