During .NET v1 days, I have tried without much success to convince colleagues to develop test-driven and automated-build work habits using the additional tools of NUnit and NAnt. When .NET Framework 2.0 and Visual Studio 2005 Team Suite came into picture, I was able to "force" my team into writing tests and provide themselves with visual testing right inside Visual Studio. I was further able to tweak project files with extra MSBuild tasks to carry out more build automation.
Of course this does not mean Microsoft has delivered perfect systems, but I believe they have taken these in the correct steps forward. With all these features baked right into the framework and products and becoming "native" it got abit easier to motion developers into better development practices.
Having long forgotten the open-source options (which I do miss), I am wondering what value proposition do the current incarnations of NUnit and NAnt hold? What case can one argue at this stage to convince a team not to use MSBuild or MSTest?
Clarification: My company is a pure Microsoft SI. Visual Studio Team Suite editions, Database Professional edition, TFS, and the like are available for our use. We do not use Visual Studio Professional edition or lesser.