You can test it online at pex4fun.com. Here are two tests, one using a function that takes built-in types as parameters and instantiates a Thing to operate on, and a second that takes a Thing input and operates on it directly. It seems to generate interesting cases for both, but it's much more easily readable with the more basic input types, as the parameters are easily displayed in the table.
Nice. If you use the second variant you have to click an a test case to see which arguments have been passed to the constructor of Thing. Just from the tabular view most test cases look identical.
http://pex4fun.com/default.aspx?language=CSharp&code=OgP...
http://pex4fun.com/default.aspx?language=CSharp&code=_QL...