This is just a guess, but I get the feeling their market share is probably not too large. We use it at my current job, and I actually enjoy using it. The integration build system combined with work item tracking and testing is nice if you have the ability to tailor your companies workflow to the "TFS way" (we are only partially there, still tracking bugs in our legacy intranet site). That said, it truly is a beast to setup/configure/maintain, to the point where I can't imagine any small MS shop bothering (esp. ones w/o a partner license and decent inhouse servers and possibly a build/deployment guy).
My last employer was still on sourcesafe (shudder!) when I left last year, and I'd bet they'll eventually go with something from SourceGear or Seapine before heading down the TFS path.
My last employer was still on sourcesafe (shudder!) when I left last year, and I'd bet they'll eventually go with something from SourceGear or Seapine before heading down the TFS path.