I've installed the new security update on many Macs without difficulty.
Having read the problems people are reporting with the new security update, I have come to the conclusion that in every case, the problems have nothing to do with the security update and indicate trouble in their computer systems that existed before. The new security update only affects QuickTime; it does not in any way, shape, or form modify any other files. So the people who are having trouble with Mail, or with login, or problems starting up, i can safely say with 100% confidence are having problems for reasons that have nothing to do with the security update.
It's like taking your car in to the shop to get a flat tire changed, and on the way home, your windshield wipers quit working. There is absolutely no way that changing the tire did anything to the windshield wipers; the timing is coincidence, nothing more.
The Flip4Mac vulnerability is not really a big deal. In order to exploit it, an attacker would somehow have to figure out a way to make you watch a specially crafted .WMV movie. There's no way that your computer can be affected unless you intentionally choose to view a .WMV movie that has been constructed in a special way to carry a hostile payload.