After the first few few hours with Mac OS 9, one thing seems clear enough: This may be Apple's most unobtrusive software upgrade yet. Install it--a blissfully short 10-minute process on each of our two test computers--reboot, and you are looking at . . . apparently the same computer.

Few signs of this new operating system, which officially ships tomorrow, lurk around the desktop. A new control-strip module sits at the bottom of the screen; the Finder's menus conceal a couple of new commands. Beneath the skin, much more is new: OS 9 incorporates numerous optimizations, bug-fixes and tweaks that should improve the system's stability. But that's not the kind of thing that can be proved or disproved in a first look such as this; it'll take sustained use over several weeks to establish if Apple has delivered on this promise. OS 9 does, however, feel fractionally faster in tasks such as switching from one application to the next.

Many of these changes break third-party programs--most significantly, the Adobe Type Manager utility that most Mac users employ to keep fonts from looking jagged. Cautions Ted Landau, who runs the MacFixIt troubleshooting site {lt}http://www.macfixit.com{gt}: "In the end, these [conflicts] will be resolved by updates to the software (many of which are already coming out). But that end is still at least a month or two away."

Meanwhile, the most interesting and useful parts of OS 9 reside under the Apple menu. Setting aside improved file-sharing capabilities and better color-matching tools, which should make life easier for graphic designers and art directors, the main attractions to home users are the multiple-users option, the Keychain, automated software updates and Sherlock 2.

Multiple users is the single biggest change here, giving different people personalized desktops on the same computer. Parents (or their gear-head kids) can give each user a different set of applications and access privileges. The most restrictive "Panels" setting locks down the computer, putting just two windows on the screen--one with allowed applications, the other with that user's documents. Setting up a machine for multiple users does involve a certain amount of tedium, as you scroll through a long list of every installed program and choose what to allow.

Unfortunately, we couldn't try out the voice-login option. It seems very cool: Record your "voiceprint" phrase four times (e.g., "my voice is my passport" or "greetings and salutations") and the system will then know you by sound. But the login-manager utility balked at recognizing the external microphone on a Mac clone, while our iBook lacked any sound-in capability.

The Keychain is a good idea badly done. It aims to consolidate your accumulation of passwords behind one master password, but it doesn't yet handle Web-site passwords without a clunky workaround. It also won't store Web passwords longer than eight characters--most of mine run longer than that. With the Keychain also comes a new encryption utility, which lets you encode individual files--say, your Quicken data or your e-mail inbox.

OS 9's new Software Update control panel, by contrast, is a good idea smartly done, fixing one of the big annoyances of life in Mac-dom: Apple's ongoing trickle of bug-fix updates. Open this control panel, click "Update Now" and it scans an Apple database for new system software (but not other firms' software). This check takes only a few seconds, although downloads will take longer. Be sure to turn this feature on--it's not enabled out of the box.

Finally, Sherlock 2 enhances the meta-search tool Apple introduced in Mac OS 8.5. Inside a new, stylized interface, it offers "channels"--your own hard drive, the Web at large, people, shopping, news, Apple, reference and one or more of your own customizable channels. Click a channel button to focus your query on that category of Web sites. But the set of included search plug-ins is often lame; "news," for instance, consists of C{vbar}Net's News.com, CNN, ESPN, Motley Fool and Quicken.com, an assortment that smacks of marketing deals.

Fortunately, the fix for that was quick at hand: Typing in "Sherlock plug-ins" in the Web-search channel quickly led to an online compilation of them. A few brief downloads later, we unstuffed the new plug-ins, and, without really thinking about it, dragged each one on top of the appropriate channel button. Click, no more Sherlock problem. That's what we like about the Mac, crashes and all: So often, things just work in the way you'd expect.

Mac OS 9, Apple; $99 ($20 rebate available to users of Mac OS 8.5, 8.6).