The issue with using random test data is the it doesn't expose your edge cases. There was a post on HN a short while back that showed why - in essence your edg

The issue with using random test data is the it doesn't expose your edge cases. There was a post on HN a short while back that showed why - in essence your edge cases sit at the edge of the normal distribution ergo they will rarely be hit by the unit tests. This leads to you doing more work, as you need to log what data caused the test to fail. It also gives you a false sense of security, since the tests aren't really doing anything 99% of the time.