Fifteen years ago today, Apple released Mac OS X 10.0, the first public release of the operating system that would become, over the next decade and a half, the software underpinnings for the iPhone, iPad, and Apple Watch.

That future wasn't obvious on March 24, 2001. Apple's attempt at a modern OS -- code-named Cheetah -- was brimming with possibilities but frequently more frustrating than satisfying. "It's not a finished operating system," wrote Macworld editor Jason Snell in his review of Mac OS X 10.0. "[It] doesn't quite work right."

Mac OS X 10.0 install discs

After watching Apple bottom out in the mid-1990s, Mac users welcomed back Apple cofounder Steve Jobs at the end of 1996, who brought with him the NeXT software that would become Mac OS X. But Mac users then had to wait more than four years while Apple engineers worked to bolt a Mac interface onto a Unix foundation. The result was Mac OS X 10.0, a wobbly release missing basic capabilities -- such DVD playback and the being able to burn CDs -- and prone to slowdowns and crashes.

But Mac users wanted to believe that Mac OS X was the way forward. Even the name of the OS became a shibboleth, marking off true believers from the unfaithful. (The "X" in Mac OS X is a roman numeral, not a letter, and those who referred to Apple's new operating system as "Mac OS eks" were viewed as illiterati.)

A handful of machines could run the PowerPC-based OS. Approved models of the time included the Power Mac G4, Power Macintosh G3, iMac, iBook, PowerBook G4 and G3 (except the original PowerBook G3), and iBooks. More limiting, the new OS couldn't be installed on or boot from an external FireWire or USB drive.

To help smooth Mac users' transition to the modern operating system, Mac OS X ran in tandem with Mac OS 9.1, which Apple dubbed the Classic Environment. With Classic, users could continue to use their older Mac OS 9.1 application while running Mac OS X.

Apple responded quickly to user concerns, and with the release of Mac OS X 10.1, Puma, in the fall of 2001, Apple has started to fill the gaps.

After that, the improvements came at a steady clip. In 2003, with Mac OS X 10.3 (Panther), Apple added the Safari browser. Mac OS X 1.4 (Tiger), introduced Spotlight. Leopard, the code-name for Mac OS X 10.5 in 2005, had Time Machine and was the last OS to run on PowerPC chips. By the time Mac OS X 10.6 (Snow Leopard) arrived in 2009, the OS was solidly running on the Intel platform, offering a full set of capable apps.

Clifford Colby follows the Mac and Android markets for Download.com. He's been an editor at Peachpit Press and a handful of now-dead computer magazines, including MacWeek, MacUser, and Corporate Computing.