In what is sure to end up as a flame war, Jeremy Miller asks the question: "is there a good reason to switch to MSBuild?". I want to throw my $0.02 into the mix.
I do not have a compelling reason to switch to MSBuild from NAnt. As pointed out by David Starr in his open letter to Scott Guthrie, what we often times see from Microsoft is a half-assed attempt to implement something that has already been evolved in the open source community without adding any value to it.
MSBuild does nothing to improve on the functionality provided by NAnt. The same way that MSUnit does nothing to improve on the functionality provided by NUnit. Many will argue that MSUnit is considerably worse, actually.
So, why would I switch to a closed source tool, when I can get an equal feature set, with greater community support and quicker response times to questions? Easy... I wouldnt.