Oddly, I didn't have any problem with the Intel Mac I ran it on, but an older G4 one had a problem booting afterwards. Usually it will boot ultimately, but might be slow. The trick is to repair permissions and have a utility on hand that can redo the "optimizing" that Software Update does at the end of its process. One or both of those pretty well cures what goes wrong with any update. (Wasn't just 10.4.9 either. A couple weeks ago I had a 10.3 machine hiccup similarly on some standard security updates and the DST update.)