Every recent QuickTime update has serious issues. I personally had to downgrade back to 7.0.1 from 7.0.2. 7.0.3 is fine for me (but not for many). I have not installed 7.0.4 because I simply have no more confidence in Apple's updates. My advice on this is simply "if it ain't broke, don't fix it"...unless you're prepared to spend the time necessary to downgrade if something goes wrong.
As always, check out
MacFixIt to get an idea of the problems people are having. Specifially,
this article has some info on QT 7.0.4.