| First a FYI that reportedly the Flash plugin version included in 10.6.4 is v10.0.45.2, which contains 32 vulnerabilities. Last week Adobe released v10.1.53.64 to address those - you can download the updated Flash version at http://get.adobe.com/flashplayer/.
I downloaded the 10.6.4 Combo updater last night but didn't update any machines right away. As usual some have had major problems and of course there's horror stories on the net (like the register's latest poke - Inevitable OS X 10.6.4 update problems surface) and threads in Apple's OS X Snow Leopard Install/Setup forums on 10.6.4 update problems, including one on OS X 10.6.4 installation restart Blue Screen (even from "Combo" updater users) that includes a Tip on Forcing a Restart (via power button held down) that helped some. But often nothing is a universal fix for everyone. (I applied the combo updater on an early 09 MBP and Mac Pro after a clean reboot - both updated/restarted ok.)
I know you're tired of hearing it, but always have a current backup before applying any update. (Time Machine at least, or preferably a verified bootable and working clone of your OS X drive.) Often (but not always) 3rd party addons can be a source or problems and it's easy to forget about some past addon/startup item, etc you installed (and may have never updated) - check the dev's site for updates/info on any issues. If in doubt, remove them.
But regardless of addons, there always seems to be a certain % of horror stories with any update. Clean restarts and verifying the boot drive before updating (and checking versions/info on any addons) is recommended (some go as far as repairing permissions before/after updates), but nothing is a sure thing it seems. There's just far too many variables in installed software, drive conditions, etc. to guarantee any update will be problem-free for everyone. (Even if the update doesn't have any known issues, which isn't always the case.)
I wish Apple would release all Security updates separately. The old golden rule "if it ain't broke, don't fix it" applies - read the docs on changes/fixes in any update and decide if it's worth updating for your needs/usage. I know a lot of people that rely on their Mac for income that avoid any updates that don't clearly address (proven by earlier adopters) their needs or existing problems they've seen. I've never updated my main work machine to Snow Leopard, as there was nothing in 10.6.x that I really needed for that machine's use.
Tonight I applied the 10.6.4 combo updater on an early 09 MacBook Pro. The only 3rd party addons on it are SIL3132 eSATA expresscard drivers and Steam. It restarted fine but didn't auto-mount the USB flash drive (that had the Combo updater DMG on it) - but removing/re-inserting the flash drive mounted it ok. I then used the combo updater on an early 09 (dual 2.66) Mac Pro 10.6.3 HD and it also updated/rebooted ok. Hopefully no problems w/10.6.4 in use, but too early to tell at this point.
Valve Warns about 10.6.4/Nvidia GPU Performance Issues: It's too late for many users, but late Friday (the 18th) Valve posted a warning to Nvidia GPU users/gamers about OS X 10.6.4:
"10.6.4 Performance Issues for Nvidia Users
The recent 10.6.4 update from Apple has noticeable performance issues for NVidia graphic chip owners running high performance games.
If you wish to avoid this, you should consider waiting to install the 10.6.4 update until Apple has had the opportunity to address this issue.
Full details of what is contained in the 10.6.4 update can be found here: http://support.apple.com/kb/HT4150.
If you have already installed this update and believe your graphic performance is affected, please contact Apple support (http://www.apple.com/support/) for details on what to do.
dberger (Valve)"
There's also a thread in the Steam forums - 10.6.4 Performance Issues for Nvidia Users. (One post there also mentioned this may not be limited to just Nvidia GPUs.)
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