A common misconception each update that appears on Discussions is that after installing an update, is that the update is to blame for problems if they happen. Nothing can be further from the truth. System software can only be tested to work with a variety of hardware and software that Apple itself supports. There are over 23,000 titles that would be necessary to test in every combination to ensure an update would work with third party software and hardware. If an update messes up your third party hardware or software, look for driver and firmware updates from the third party. Third party is anyone other than Apple. You may also find permissions, preference, font, or cache corruption related to specific software or drivers which cause the issue. Suffice to say, should you have problems with an update, don't be too quick to blame the update itself. This also says you should backup your data at least twice before updates, as the possibility might be your hard drive is dying, and the update may not be compatible with an external hard drive. So to take care of both possibilities backup your data at least twice so you won't be caught in a struggle to rescue your data after an update, which has been seen to happen before. Often what is wrong is simply the need to repair permissions and apply the combined update from http://support.apple.com/downloads/ or the hard drive is too full. Ask here on discussions before updating for other precautions you can take if you suspect your machine isn't entirely healthy or need to learn how to make a system check before updating to avoid problems, or need to know how to backup your data. Many people know how. Don't assume the update has been tested with your third party hardware or software, and take necessary precautions before each update. Check with third party vendors if the update has been tested, as well as ask on here with any additional software or hardware you might have whether anyone has tried it with the update.
In addition, reverting back to a previous OS version is often only possible if you backed up your data properly. So learn how to backup your data so you can revert easily, if that is a concern for you.