One of the most exciting things that a playwright can do is to show you an unfamiliar way of life. A play that succeeds in doing so can be forgiven any number of theatrical sins. "Stick Fly," in which Lydia R. Diamond puts America's black upper class onstage, fills the bill on all counts. Yes, it's a mess, but a fascinating one, well directed by Kenny Leon and performed with total persuasiveness by his ensemble cast, and the best parts are so good that you'll be glad to forgive Ms. Diamond when she goes wrong.