I've been using the 9.0.4 update since Thursday, and I've got nothing but good things to say about it. My SCSI scanner works again, general processes seem faster, and seems more stable.
I think that most problems are related to how you install the update. I learned long ago that it's often a very bad idea to install a [system] update without first zapping the PRAM (hold "apple+shift+P+R") about four times before allowing the computer to start with extensions disabled. Then after the installation finishes, restart, them rebuild the desktop. By adopting this technique, EVERY system installation or upgrade has worked as intended, in my expeience. Before using that method, upgrades were an "install and hope for the best" type of occurance.
Just my suggestion...
oh, running OS 9.0.4 on a Performa 6400.