Itt just means it's a public beta... not the final release. Bascially their way of saying that if you rely on Quicktime for work, you shouldn't use it. Otherwise, go for it.
This is pretty buggy for me. It opens every new movie in a new window even thoughI have it set to open them in the same QT. It's also asking me if I want to save changes everytime I try to close a QT movie. Anyone else having problems?