As with any system update, there are incompatibilities of one sort or another. Most of the reports are anecdotal, about isolated incompatibilities with this program or that hardware. Quite often, a clean installation of the original operating system and running it again will eliminate the troubles. But even Apple's own technical notes show a normal number of problems. They are recommended reading, since the information that comes with the update itself isn't near as complete.
Some reports speak of connection problems involving USB modems.
I've seen reports of various and sundry problems with a fiar number of products, but when you're dealing with such reports, it's difficult to get to the bottom of the situation without actually examining an individual's particular setup. That's one of the problems in dealing with Web-based reports of an incompatibility unless the sheer number of similar reports show a frightening trend. For most users of Mac OS 9 or 9.0.4, the answer is an unqualified yes. If you're still in the Mac OS 8.x zone and have been fearful of upgrading because of all the troubles the arrival of Mac OS 9 wrought, you can be a little more secure in the knowledge that much of the grief has been massaged out of the system by now. You can download your copy direct from Apple's Web site .