Pex comes with a Visual Studio Add-in and samples which only work with Visual Studio 2008 Professional (or higher).
Does not require Visual Studio. If you have Visual Studio 2005 or Visual Studio 2008 *Express* or even no Visual Studio on the machine, Pex also works from the command line. The new Code Digger experience requires Visual Studio 2008 Professional, or better, with support for the MSTest unit test framework. Otherwise, you will see an "unexpected tool error" in this release.