Update 2007-11-30: PC Magazine has jumped into the fray with the op-ed piece Leopard is the New Vista, and It’s Pissing Me Off, noting:
Let’s see, Tiger crashed—oh yeah, NEVER. Ten months and I’m installing everything from production-level Office for the Mac 2004 to 0.x releases of VLC, Seashore, and Ecto—even betas of Firefox and Parallels. Whatever my nerdy little heart desires. I’ve had those early apps crash, but Tiger never faltered.
A month of using Leopard with the same software I had under Tiger and the OS has dumped six times. That’s six cold reboots for Oliver. Apple isn’t even honest enough to admit that Leopard is crashing: The OS just grays out my desktop and pops up a dialog box telling me I’ve got to reboot. Like the whole thing is my fault. I even snapped a picture of it. After all, I HAD PLENTY OF CHANCES!
I think there’s one big difference between Leopard and Vista: Apple has a chance in hell of fixing Leopard. Microsoft, I think not so much.
Update 2007-11-28: Seems the primary source of the Atheros airport driver death problem has been determined, according to Apple Insider:
The exact behavior of affected systems can vary slightly, but appears to largely be connected to the computer’s power supply. While the connection remains largely stable when plugged into an AC adapter, switching to battery power renders the wireless link intermittent and in many cases drops network access entirely when idle.
Amusingly, I proved confirmation of this myself moments ago when I accidentally knocked out the mag-safe connector and… my access died and then my machine kernel panic’d (the mag-safe connector remains one of my favourite laptop features of all time, honest).
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After a month of using Leopard it saddens me to say that this is the first release of any Mac OS that makes me regret having installed it. I’d roll back to Tiger in a heartbeat if I could. If you haven’t updated yet and you’re on the fence I recommend waiting at least a couple more point releases before taking the plunge because this OS crashes, and when it goes, it goes hard.
Empirical evidence speaks louder than anecdotal so here’s the crash report windows for the latest four application deaths: Airport (the first two, from different crashes - a seemingly common occurrence), Front Row, Software Update and Transmit (you’ll note that three out of four of those are Apple apps).

The net result has been a whole lot of these, roughly once every two days:

(Yes, all of these crash reports have been dutifully submitted to Apple.)
Ordinarily I’d be tempted to write it off as an isolated occurrence, perhaps something to do with this machine, but I see the same sorts of things happening on my iMac. Eventually it dies a hard, slow death the worst being the complete unresponsiveness of all applications.
Every now and then on launch an application will bounce for a long, long time and then fail to start though the icon appears in the Dock - evidently inhabiting some sort of semi-started purgatory. After that no applications can be quit or launched successfuly. I suspect this is the result of a failure within launchd somewhere but the net effect is that the OS becomes useless.
My gut impression is that Apple bit off more than it could chew with Leopard and made too many fundamental changes to the internals that haven’t been tested nearly enough. The things I really like about Leopard, like Spaces and Time Machine, are stand-alone applications at best and not fundamental improvements to the state of the art worthy of this grief.
The net result is a half-baked, rather unstable OS that I just cannot trust. My laptop and the iMac are now backed up to different external drive hourly by both Time Machine and SuperDuper!.
As such I’ve geared myself up to set aside a day next week to do a complete clean re-install of everything to see if that helps.
I cannot tell you how much it saddens me to be cursing the daily operations of my Mac as much as I would a PC.
Update:
Seems some commenters have some strong feelings about how it is in effect my fault these are happening so I thought I’d link to some other folks noticing these issues. Share the comment rage people, don’t waste it all on one blog.
100+ messages about the Atheros kernel panic situation.
Another thing I’ve noticed is that program stability in Leopard is definitely not up to what I’ve come to take for granted in Tiger, and I’ve had a more application crashes in two days than I’ve probably experienced in the past six months with Tiger. Some might dismiss this as less than optimum program optimization for Leopard, but that doesn’t explain the Finder locking up, and Pixelmator 1.0.2, another app. that has proven crash-prone in Leopard, was just released last week. And those aren’t the only ones that have crashed. I’m becoming a lot more re-acquainted with the Dreaded Spinning Beach Ball of Death than I ever wanted to be.
- Learning to Love Leopard
Anyway after this installation everything seemed fine and Apple had just released the 10.5.1 update. Then the lock ups started, intermittent keyboard lock ups, programs just stopped responding (force quit wouldn’t quit them), trying to mount DMG’s or extract zips failed with all manner of errors. Bizarrely enough a reboot resolved all of these- until they started to happen again. The my iDisk started to complain, by complain I mean I noticed it wasn’t doing what it should. Finally this was just one hard reboot too many for Leopard and I got a massive corruption of the drive - it would crash as soon as it tried to load the boot loader.
- Leopard - My painful experiences… So far
Fight!