It's been two years, five months and 26 days since Apple last released a new operating system -- the longest gap between releases since the company first released Mac OS X six years ago. Mac OS X 10.5, better known as "Leopard," hits store shelves six months after it was initially expected -- and more than a year and a half after Apple CEO Steve Jobs first showed it off in mid-2006.

So, is it worth the wait? And, more important, is it worth the US$129 price tag? For Mac users, the answer to that question depends on whether they're happy with the current OS, Version 10.4 "Tiger," whether they're using hardware that can run Leopard, and whether they're brave enough to install a .0 version of any operating system. (Just ask Windows Vista users about that.)

What's new in Leopard? A lot. From the unified interface (goodbye, brushed aluminum) to major under-the-hood changes, to wholly new apps, Leopard is a substantial, albeit evolutionary, advance for Mac OS X that builds on a solid foundation and adds a modicum of eye candy to reinforce the notion that this is something new and improved. It's also fast -- especially impressive given the new graphics sprinkled throughout the OS.

While Apple lists more than 300 changes for the OS, most users will be focused on the biggies: Time Machine, Spotlight, Quick Look, Spaces, Parental Controls, a revamped Finder and Dock, an updated user interface, much-needed tweaks to programs like Mail and iChat, and behind-the-scenes changes that should make it easier for developers to improve their own third-party applications.

As with all new operating systems, Leopard is also likely to bring some unwelcome changes that could prompt some would-be buyers to hold off on the upgrade, at least for now. Although Leopard is designed to work just fine right out of the box on all of Apple's Intel-based hardware and on Power PC G5 machines, owners of older G4 laptops and desktops -- those running at speeds of less than 867 MHz -- are out of luck. Leopard also requires at least 512MB of RAM and a DVD drive.

If your computer doesn't meet those specs, it's time to upgrade your hardware or stick with Tiger for now. And if you're still running Mac "Classic" OS apps, forget it. Leopard drops support for what was once Mac OS 9.

For enterprises, a new operating system almost always means compatibility issues with at least some mission-critical apps. The Leopard client OS and Leopard Server, which offers its own set of much-needed improvements, is likely to be no different -- no matter how hard Apple works at backward compatibility.

IT departments will want to do extensive testing before rolling out Leopard, because it's almost certain that some software -- especially third-party applications -- won't work right away. (For example, FileMaker is recommending that people hold off on using its software with Leopard until the company releases compatibility updates. And Mozilla reports that some Firefox Add-ons don't yet work right in Leopard, although the browser itself does.) Even so, most major applications and software drivers appear to work as they should, based on testing of the OS by Computerworld and reports from those who worked with builds when it was under development.

Given the hype surrounding Leopard -- and Apple's recent success in bringing new users into the Mac fold with its popular iPhone and iPod Touch -- millions of users will likely ignore any cautionary notes, rip the plastic wrap off the box and have Leopard installed on their machines as quickly as possible.

For those not sure what the big deal is or whether they should make the leap to Leopard, we offer this in-depth overview of the major changes and new features in Mac OS X 10.5.

Over the next week or so, we'll publish extensive, hands-on reviews of Time Machine, Apple's take on backup software; Spotlight, its supercharged search app; Leopard's user interface changes; additions and tweaks to Mail, iCal and .Mac; and not-so-obvious changes aimed at developers; along with hits and misses, and, perhaps inevitably in the IT world, a comparison with Microsoft's Windows Vista.