There's a lot of truth to this. I work in a project with a lot of separate assemblies, because we have many applications that share similar functionality. Unit tests are critical to making sure I don't break something for one application while making a change for another. However at the same time, It can be a real pain to load the entire application to test it. Unit tests have actually increased my productivity in some areas where doing a test has about a 1-5 minute overhead (compiling than loading a file etc)