DirectX Happy Uninstall is the name of the program and from what I know, it is long overdue. There are a lot of reasons to roll back DirectX; probably as many reasons as folks use to justify upgrades. Most of us just don't need it and we may not want to dedicate the resources it takes to it. It's a personal decision.
First reports, like this one in The Inquirer, are good. I have chatted with a couple happy users and don't know of any unhappy ones. It does not yet work on Vista or DX10, but for XP users, this is a good one. The price is right, too.