I am part of a team that is an active participant in multiple TAP programs. We've had great success with all of our JDP/TAP programs. We were in the Windows 2003 (Whistler) JDP, the Windows XP SP2 TAP, and we're currently in the Windows 2003 SP1 TAP and the R2 TAP. It is critical that you have executive backing and have the resources to commit to the effort.
Our executive management has seen the benefits of participation and active encourage it. Of course we test, test and test again in the lab before we attempt to put any production resouces on beta code. Even then we start small. But we've operated our large centralized file cluter on Windows 2003 since RC1. That server services over 12,000 clients. Not one hiccup the entire time.
In my experience by the time the TAP customers get their hands on Beta code, it's already pretty solid.