Many of us at Supercede use GHCi directly when developing Haskell projects. It runs the local development server, and the automated test suite, and serves as a scratch pad for ad hoc expression evaluation and type inspection. We make heavy use of integrated tests with the yesod-test framework. One of our projects now has more than 500 tests. Running the tests in GHCi takes about 150 seconds. Not terrible, but not great either. Certainly not fast enough to encourage developers to practice TDD ...
