I am from Typemock, and I won't address the "Too Powerful" comments (although it's beyond me why people wouldn't want to use the best tool for the job).
Here's how Typemock Isolator works. Have you ever used a performance profiler? Isolator is a profiler. It hooks into the CLR and within the test run time, it changes methods. When a method gets JITted, it changes it, so when the method runs, before executing the original code, it asks: should I run it as originally intended, and if not, what should I do? And because of this specific technology, it can mock any .Net method and technology. That's it.
When you set a behavior using the API, the answer to the questions now becomes intersting, and changes the behavior at runtime of that method. Simple, but under the covers it's a lot of work:)
Isolator comes with a VS AddIn to make running the tests seamlessly within VS, and with a command line tool and MSBuild or NAnt tasks for usage in a build server.
I'll be happy to answer any other questions you may have.
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