Unlike the Windows world, where operating systems upgrades are sources of frustration and loathing, among Mac users upgrades are met with enthusiastic interest. I've been using Leopard (Mac OS X 10.5) for a few days now.
The performance improvements promised by Apple are real. Everything feels snappier. Spotlight is actually usable now.
My favorite new feature is Spaces. Some say "Big deal. It's just a virtual desktop manager. UNIX workstations have had those for years." True, but it is an improvement over the original Exposé feature. Unlike other virtual desktops, it is well-integrated into the rest of the UI. Dragging live windows between virtual workspaces is really cool.
Time Machine is pretty cool too. Again, some would say "Big deal. It's just a backup/restore application. I can do the same thing with rsync." What makes Time Machine special is its simplicity. You plug an external drive in, and the Mac asks "Do you want to back up your main drive to this drive?" If you answer "yes," then that's it: you now have automatic hourly, daily, and weekly backups. Unlike other backup systems, Time Machine keeps all these backups available, but conserves drive space by not making copies of files that have not changed from one backup to another.
Time Machine is one of those amazingly great things that seems obvious, now that somebody has done it. I expect Time Machine clones to appear for Windows and UNIX very soon.
I do have some complaints about Time Machine: the UI is a little hard to figure out the first time you see it (and there is no menu bar or online help available in the app), and my MacBook CPU usage goes to eleven for a couple of minutes every hour while it makes the backups. I may turn off the automatic backups and switch to manual backups (right-click the backup drive and choose "Backup Now").
The new version of the Safari web browser is a lot more usable than the previous version, but I went back to Firefox after a few days. Firefox has more "power-user" features than Safari does, and I can't live without them.
I have mixed feelings about the "Leopard look." On one hand, brushed metal has pretty much disappeared, so we can rejoice. But there are other things that, while they look cool, actually make it more difficult to see important information: translucent menus, the 3D Dock, subtle folder icons, too-dark windows, etc. But it's not as bad as Vista.
On the whole, it's a solid upgrade. In a way, it is a bit of a letdown, because it is really just a polishing of an already-good system. Of its touted "300+ new features," few are going to change the way one uses their Mac. I haven't found anything that makes me say "Wow!" but there are a lot of little new things that make me say "Hey, that's kinda neat."
For a good in-depth technical review of Leopard, see the Ars Technica review. From that review, it looks like it is a great time to be a Mac developer—lots of cool new APIs and debugging aids. (Unfortunately, I'm still a Windows whore.)
To sum everything up: the upgrade was definitely worth $129 and a few hours of time. My only regret is that I didn't buy the "family pack" so that I could also upgrade my old iMac G5. Is it time to buy a new iMac?