Among the Microsoft certifications, the Microsoft Certified Master is the apex of all of them except maybe the Microsoft Certified Architect. But you can't run down to your local bookstore, grab a few books, and go take a test to earn it. It requires 3 weeks of study at Microsoft along with several exams/labs. As a result, there's not a whole lot of information out about it other than "it's hard." Brent Ozar is matriculating through the current MCM: SQL Server class and has been blogging his observations and thoughts. He can't go into the specifics of the materials or the questions, because those are under NDA, but he does give a lot of great insight as to why this program is so valuable, in fact, more valuable than just any old certification. You can find the series here:

Brent Ozar on Microsoft Certified Master: SQL Server