Starting today, Microsoft will try to reinvigorate its flagging Windows franchise, reignite falling PC sales, and maybe even save its almost invisible phone business with the release of just one product — Windows 10.

At its heart, Windows 10, which will begin rolling out gradually as a free update, is a rescue mission. It’s an attempt to almost fully backpedal from its 2012 predecessor, Windows 8 (they are skipping 9), which was a radical effort to redefine the way Windows looked and worked. That experiment failed to win the hearts and wallets of consumers, and is estimated to have only about a 16 percent share of global PC users.

Instead, Windows 10 more closely resembles an even older version, the 2009-era Windows 7, with a dash of the Windows 8 look and feel retained. Unlike Windows 8 at its launch, the latest version of Windows will boot into the familiar desktop metaphor, and boast a full, working Start menu.

There are few wholly new big features, and some of them are catchups with those on Apple’s Macintosh OS X.

Also, the near-final build I’ve been testing proved surprisingly buggy. In particular, I had trouble with Windows 10’s sexiest new feature, the voice-controlled Cortana intelligent assistant — Microsoft’s answer to Apple’s Siri — which has migrated from Windows Phones to the PC.

Still, some of the new features are promising, the balance between old and new styles seems right this time, and — if the bugs get erased — Windows 10 would be a good choice for Windows devotees.

However, it’s just okay, not disruptive. It’s perhaps what Windows 8 might have looked like if it had been evolutionary, not revolutionary. I doubt it will convert many Mac owners, spur a shopping spree in new PCs, bring in droves of new developers, or save the Windows Phone.

And I advise would-be upgraders who aren’t enthusiasts to wait to upgrade at least for a few months, until the product is more stable and reliable.

Microsoft says that Windows will now be treated as a “service,” with frequent small updates to quash bugs and add features. So this may be the last release of Windows with a formal new name.

Easing the Confusion

Windows 8 confusingly jammed two different interfaces, with two different types of apps and two different optimal input methods (touch and mouse) into the same OS. It eliminated the Start menu and booted instead into a Start Screen with tablet-style apps represented by tiles. The standard desktop, and its standard apps, were demoted.

In Windows 10, PCs boot into desktop mode. The tiled tablet look has been demoted to a section of the restored Start Menu. Or you can decide to switch to Tablet Mode, where it will dominate. But it’s your call.

What’s more, the tablet-style apps run inside windows in the desktop mode, just like traditional Windows programs. A new type of “universal” app will supposedly be able to run, in the appropriate mode, on all forms of computers — traditional desktops and laptops, hybrid laptop-tablets, pure tablets and even, later this year, Windows phones.

Cost and Strategy

Microsoft is giving away Windows 10 as a free upgrade to users of the latest revisions of Windows 7 and Windows 8. The free offer lasts up to a year from today, but it won’t be immediately available to all. It will begin with beta testers (so-called “Windows Insiders”) and then spread to people who have reserved copies; then, on to others. A prompt will pop up on PCs that are eligible to upgrade.

The hardware specs for running Windows 10 are identical to those for Windows 7, but it still may not work well on older machines with slower processors and other limitations. The installation app will check for compatibility.

Still, Microsoft hopes that hundreds of millions of people will upgrade. That’s a reversal of its longtime strategy, in which it paid little attention to upgrades, and saw new versions of Windows as mainly a way to sell new PCs.

The reason? The company hopes that if it can get enough PCs on Windows 10, developers will be incentivized to write the new universal apps. And, since these will also run on Windows Phone, it will help revive that failed mobile platform, which has struggled to attract popular apps.

If you don’t, or can’t, do a free upgrade, you can buy a new PC, or Microsoft will sell Windows 10 starting at $119.

What’s Old Is New

In some ways, the biggest new feature is an old one — the return of the Start menu. As in the past, it’s on the lower left of the screen and lets you launch apps and settings easily, and shut down the PC. But now it includes a mini version of the Windows 8 tiled Start Screen, which you can expand.

This menu lists your most-used apps, those you’ve chosen to pin to the menu (as tiles) and even all apps, if you care to see them. Settings have been simplified and enhanced but — confusingly — for some things, you still have to use the ancient Control Panel, which is still there. Microsoft says it’s working to eventually get rid of the Control Panel, but not soon.

Listen to Me (Some of the Time)

The most notable brand-new feature is Cortana, the combo search box and intelligent assistant. By typing in a box labeled “Ask Me Anything” at the lower left of the screen or speaking a question using a microphone icon, you can look for files, launch programs, play songs, get information from the Web and more.

For instance, in my tests, it could tell me the weather, or how my calendar looked, or how many people live in Kansas. It could find various documents by searching for words within them, and set reminders. You can even opt to have it listen for a trigger phrase (“Hey, Cortana”) and just speak questions or commands to it without tapping or clicking on the microphone icon.

The problem was that “Hey, Cortana” failed on me about half the time, and the microphone icon also failed from time to time. I got messages saying “I didn’t get that” or “something went wrong” or “the Internet and I aren’t talking right now.” At one point, I even got a pop-up warning on my PC saying the microphone wasn’t designed for Cortana. And this was on a new Dell XPS 13, lent to me by Microsoft for testing, which costs more than $1,500.

Cortana needs work on the PC in Windows 10.

Navigation and Notification

I very much liked two new navigation and notification features. One, called Task View, appears when you swipe right on a touchscreen from the left edge, or tap or click on a button at the left end of the task bar. It shows all your open windows and lets you select among them. But it’s a catch-up feature: Apple has had something similar, now called Mission Control, for years.

The other, called Action Center, appears when you swipe to the left from the right edge of the screen, or tap or click an icon near the right edge of the task bar. This shows notifications, like new emails, Facebook posts and similar things. It also has quick settings, including the ability to switch to tablet mode. Alas, again, the Mac has had something like this for awhile now, which also appears at right side of the screen.

Other New Stuff

Another of the big new features is a new Web browser called Edge. Its main feature that isn’t a catch-up is the ability to draw or scribble on Web pages and then send these annotations to others. This is not something I believe most people would use frequently. A full review of Edge by my colleague, Lauren Goode, is here.

Gamers will love a new Xbox app, which can stream games to a properly configured PC from an Xbox One, and includes a screen-capture feature that takes images and videos of the action. I couldn’t test that.

There’s also a new function called Windows Hello that lets you replace a login PC password with facial, iris or fingerprint recognition. But my Dell test unit lacked the hardware needed to test that.

The built-in apps — Mail, Calendar, Photos, Maps, Music and more — are much improved from their counterparts in Windows 8. But they still need work, especially Mail. It lacks a unified inbox and unified unread view for all your accounts. And, just like Apple’s built-in Mail app, it had periodic trouble with Gmail.

Continuum

Though I couldn’t test this, Microsoft now has a feature called Continuum, which allows you to switch to tablet mode — but doesn’t force you to do so. For instance, if you have a hybrid PC, and separate the screen and keyboard to use it as a pure tablet, tablet mode can be turned on, and you can continue your work where you left off.

Downsides

If you love watching DVDs on your PC, or still use floppy drives, you’ll now need third-party software for that.

Also — and this may not be the fault of Windows 10 — I still found the touchpad on my expensive test Dell laptop to be slow and jerky compared with the one on my Mac. This wasn’t supposed to be the case, since the machine has a new so-called “precision” touchpad promoted by Microsoft. But the touchscreen worked very smoothly.

Bottom Line

Windows 10 will finally give the great majority of PC users, who still use Windows 7, a familiar but improved upgrade. However, by making that upgrade free, Microsoft may be dampening, not boosting, the market for new PCs, at least in the short run.

I regard Windows 10 as a solid, evolutionary operating system that’s likely to be a good bet for people who like Windows. But don’t upgrade until more of the bugs have been worked out.