I admit it, I was one of those people who bought Mac OS X 10.5 Leopard on the first day. Here are my first impressions.

CRITICAL BUG ALERT!
First, if you're upgrading to Leopard, there is a critical bug in the system which shipped on the DVD. Under some conditions, this bug can cause the Keychain manager to wipe out the user password, making it impossible to log in. If the user account is also the admin account (as is the case on most single user machines), this will also make it impossible to install system updates, manage other accounts, etc.

After upgrading to Leopard, run Software Update immediately, before running any other programs, especially Mail and Safari (both of which make heavy use of the keychain). Apple has published a fix, and running Software Update right away will prevent the bug from affecting you. You should "Cancel" any requests to access the Keychain before installing the update.

If you do get bit by the bug and find yourself unable to login, it is possible to reset your password with about five minutes effort. Here are the official instructions for recovering a lost password after a Leopard upgrade. It is not difficult, but it requires booting into UNIX single user mode, and entering some obtuse command lines. Just follow the instructions precisely as written and you'll be fine.

Apparently this bug affects accounts which were originally created on 10.2.8 or earlier, even if that account was originally created on a different machine. So if you've been migrating your accounts through a couple generations of Macs, there's a good chance you're vulnerable. She Who Puts Up With Me is a software QA professional, and she said she can see how this bug slipped through: there's a limit to how many generations of hardware and software you can test through, and this is one of those crazy edge cases which is rare in the development environment, but not that unusual in the field.

Other Impressions
Overall, this is probably the "dirtiest" OS X release I can recall, and I've been on OS X since 10.1. In addition to losing my login password, several of my system and Mail preferences were reset (minor inconvenience), permissions for personal web sharing got screwed up (still not fixed yet), and I had to reset my local WiFi network settings (also a minor inconvenience). In other words, don't plan on this being as simple of an upgrade as earlier version. You will have to do some tweaking.

That said, I also think this is one of the most worthwhile OS X releases I can recall. It's worth the nuisance (and in my case, pain) of the upgrade for a single new feature: Time Machine.

I've been struggling with backups on my computers for years, trying to find a system which would be (a) painless, (b) comprehensive, and (c) completely transparent. Time Machine delivers on all three. Just turn it on, and that's it. Forever more, the system will keep incremental backups every hour, and give you a slick tool to recover lost files (or the entire system, if it comes to that).

My one complaint about Time Machine is that the user interface is really cheezy. On the other hand, as John Gruber argues, that might be a good thing, since the cheez-whiz UI will get people to actually turn it on, and once on, it will run silently in the background forever, defending the user against hardware failure and accidental deletion.

As an aside, since Time Machine backs up everything, this is likely to hand a new forensic tool to those investigating computer crimes and kiddie porn cases, I'm just sayin'. You can specify a list of files and folders to exclude from Time Machine, however, so if you have something you don't want the FBI to know about, you should take advantage of that. The "Secure Empty Trash" feature will not remove the Time Machine archives--though it probably should.

Anyway, I went out and bought a cheap 1TB external drive array (less than $300), divided it into two partitions for my laptop and my wife's, and installed it as a shared drive on our iMac. The shared partitions work perfectly as backup drives, and now whenever our laptops are on the home network we have backups. Utterly painless: we don't have to do anything other than connect to the WiFi.

Plan on letting the computer run overnight for the initial backup, though. It takes a while.

One feature I would like to have in Time Machine is the option to create multiple backup volumes. My laptop spends about as much time at the office as at home, and it would be useful (not to mention good practice) to have a backup drive in each location. Since Time Machine is so painless to use, I can see this as a very reasonable thing to do. However, as near as I can tell, Time Machine doesn't permit more than one backup volume.

Other Nice Stuff
Since we have four Mac OS machines in our home (laptop for each of us, Mini for the kids, and an iMac which we use as a home server), networking and sharing is a fairly big deal. I'm really happy to see the return of shared folders/drives. What's more, the interface for file sharing is simpler and easier than ever.

Having a self-cleaning Guest account is really nice. Haven't used it, but I'm sure I will.

I haven't had a chance to play with the new Parental Controls yet, but I'm sure I will. That feature gets a lot of workout on the kids' machine.

I also haven't had a chance to play with spaces yet, but that's also going to be a useful feature. Now we just need someone to hack the motion sensor on newish laptops so you can change screens by smacking the side of the computer.

On the downside, I really don't care for the translucent menu bar, and the glowing dot on the dock for active apps (replacing the black triangle) is hard to see. Also, having open Finder windows seems to mysteriously suck CPU from time to time. Closing the window fixes the problem, but I'm not sure what exactly the processor time is being used for.