Recently I've downloaded the Mercury Quality Center 9.0 trial version to give it a try.  So far I like the Test Director.  It's pretty straight forward and very user friendly.  I am following the Test Director manual.  One of the feature I like about Test Director is there is a link between the tests and the requirements.  Hence some graphs can be used to show how many percentage of the requirements are covered by the tests.  If there are any changes to the requirement, we can know which tests are affected and hence needed a change.  Still more to read and more hands on...

Please share your ideas if you're using Mercury Quality Center.