Here are my first impressions:

• I don't care for the Aurora picture that's the "theme image" for Leopard. What about a real fierce Leopard instead?
• I honestly don't know how they do this, but every single OSX release since the first has felt snappier than the previous one. When have you ever, ever, thought your computer was running faster after an OS upgrade? They did it again! Hats off to you Apple engineers.
• Java is broken. Nothing fundamental, but the nice graphical enhancements that they added don't make up for many, many annoying problems such as a repaint problems in virtually all Java apps that I tried.
• Time Machine is the most significant innovation since the invention of backup. Here's a piece of backup software that's actually fun to use and that mere mortals can understand. Now if I could back up to my RAID-5 server via NFS instead of just an external drive ...
• Partial translucence seems to be the default now. Takes a bit of time to get used to, I guess ...
• Java apps do not behave like they should with respect to "Spaces". When selecting a running app via Apple-Tab, it doesn't switch to the right Space as needed. Spaces works very nicely for all others.
• I like that RSS feeds are in Mail.app now. I don't like that managing them is very very limited. What about: sort by author, or a hierarchical folder structure like anything else?
• Stacks are a great time-saver.
• Notes and to-do's in Mail.app is weird but probably a time saver. But what about filtering options such as "show me only to-do's that have not been done, or only those due in the next week?
• I keep getting an error dialog about an upgradedb having crashed. What the heck is that? Why does it crash?

Overall: very much worth upgrading. Time Machine itself is worth more than the upgrade.