Perian has never worked for me. I can't get Xvid or MKVs to play in QT. VLC is the only player I've found for OS X that actually plays everything.

I already have Perian. It's not supported well imho. Just try to open a 8gb mkv and it broke QT.

It didn't break Qt, it broke Perian.

That being said, I've got a couple of 8GB MKV's and they all work perfectly. So caveat emptor and all that ... :P