The Protestant Reformation was not a perfect movement, but it certainly did a lot of good. Specifically, God used it to wrench the biblical Gospel out into the open. The Roman Catholic Church was no longer allowed to shroud the Gospel of grace under its false, works-based teachings.
Martin Luther's nailing of his 95 Theses to the church door in Wittenburg, Germany was a significant event. It occurred on October 31st, 1517 (as an aside, next year will hold lots of 500 year celebrations of Luther's act). However, it wasn't the most important thing to happen. Rather than one big thing, the Reformation was a conglomeration of many different, semi-related little things that happened over the span of a few hundred years. In the end, the true Gospel was freed from the clutches of those who hate it.
If not for the Reformation, I doubt that I would know Jesus Christ as Lord and Savior. That's why I cherish the Reformation.