Well, today at around 11.30 AM GMT Leopard was delivered to my doorstep, 7 hours before it was released. Needless to say, I played with it and here are my first impressions.

The installation was very easy, few options, just an "insert disk and click go" operation. There was no need to backup (although recommended) and it just upgraded everything beautifully.

The first thing I did when I logged on was to check that all my applications worked, and I'm glad to say that I haven't noticed any problems with stability, which is nice, since this is 10.5.0 and has a little leeway for problems before the next minor update.

So I logged on and checked my main applications. Then what? Well, everyone who is into Macs will probably know about Leopards key features, but I'll start from the top. The first thing you notice is the dock. It's 3d! Little lights come on under each open application; it's very sleek. In fact, the whole interface has been given a makeover and looks fantastic with new icons, new colours and a new menu bar.

The first thing I did was to open Mail.app (to check my email of course). It asked me to upgrade my mailboxes which took under a second (for 666 messages). The new Mail.app is excellently made, with notes and to-dos, which is one thing MS Outlook always had over other mail programs. It automagically syncronised all my to-dos (using iSync) with my Sony Ericsson k800i so everything is up to date.
Then I had to check out Safari, which is also very nicely crafted, with some nice new features. WebClip is where you click a button on the Safari toolbar and it allows you to crop an area and export it for display on your dashboard! I haven't found much use for that yet, but I've only had it for a few hours! I also noticed that I can drag tabs and create new windows, but you might have been able to do that in Safari 3 beta.
Then I tried out the new Finder. I have to say that it has everything Finder has been lacking recently. The new sidebar is a good sight more elegant with smaller icons and sections for "Devices", "Shared" and "Places" making it much easier to navigate. On top of that, cover flow is a whole new world. On iTunes I never really used cover flow, because I found it annoying, but in the Finder, it's great, especially for folders with pictures of presentations etc. QuickLook allows you to preview almost any file I'm told, but I haven't really had an opportunity to test that very much, although I'm pleased to say that it probably can, from what I've tried.
On to the desktop, and one of the most important, and most useful innovations here are stacks. Stacks allow you to place folders on your dock that fan out into a stack of files when you click on them. For downloads this is especially useful, and Safari is automatically set up to download into your "downloads" stack.
Then I took a look at Spaces. Spaces are like the virtual desktops that Linux has had for years. Unfortunately, although I would like to use them more, I haven't yet managed to find much use for them yet, since I use two screens, but on the move (I use a MacBook Pro) it could be very useful. They are easy to use, however, and you can designate specific spaces for certain applications so that, for example, Mail.app will always open in Space 3 or something. You can easily navigate the spaces as well.
Once I'd partitioned my external HD, which is 320GB, I decided to check out the cool looking Time Machine. Time Machine is Apple's own backup utility, and looks very promising. It automatically backup to the Time Machine Hard Drive every hour for 24 hours, every day for a month and weekly until the disk is full. It has an incredibly easy-to-navigate system where the leopard desktop disappears, Finder opens and you can see the past stretch far into the distance.
The final thing I really noticed was the new look of Front Row, which is now faster and very, very pretty.

Overall I'd say that the Leopard upgrade is brilliant, faster, prettier and just more functional than ANY other operating system out there. I know, since I've been a Windoze, and then an Ubuntu user before switching to OS(e)X. The only problem I've had with the Leopard update is its detection of wireless networks.

Anyway, I hope that was relatively informative, and will get you to switch.