Motivated by the "TypeMock Contest: Win User Group Funding for a whole Year!", I downloaded and installed Typemock Isolator and ran through a couple of exercises to review how it works.
I find this product will be essential for someone who needs to add tests to the 'giant ball of mud' project. Isolator, by name alone, allows you to write tests which you can isolate calls to methods or just completely override entire calls, such as one to a messagebox. It also allows you to inject dependencies into a code base with no IoC container by overriding the creation of dependent objects. The real power with Isolator is that you do not need to change your code base to make it testable.
If your working on a new project and have complete time and control to code however you want, you can certainly get around without using this product. However if those niceties are not available (and even if they are) Isolator can make your TDD life significantly better.
