When your child tries to access a blocked website, the OS displays a polite message and offers to unblock it if one of the computer's registered adults provides a username and password. Leopard also logs the fact that one of the parental-control rules was violated. I'm not sure what a parent would do with this information, but it's there if you want it.
Aside from those two advances, most of the other Leopard improvements are little more than apple polishing. If you are a fan of the "smart folders" feature in iTunes, which lets you automatically see your top-rated songs or most recently played videos, you'll feel completely at home using the same feature in Apple's Finder, Mail Client, iPhoto, and other applications. These smart folders are also integrated with Apple's Spotlight desktop search: click the magnifying glass in the upper right-hand corner and type someone's name, and a new finder window will appear showing the mail messages, documents, and calendar and address-book entries that contain the name.
There are lots of other clever features sprinkled throughout 10.5. For example, Leopard now has a "Back to My Mac" feature that lets you set up your home computer so that you can remotely access its desktop and files, even if it's trapped behind a firewall (provided that you have paid your .Mac subscription fee). You can attach your Mac to an HD television set (all iMacs now feature DVI output) and use it to play DVDs. You can preview a file before you open it. You can create notes and to-do lists and store them in your mailbox (which means that they'll sync across multiple computers if you are using Exchange, IMAP, or .Mac to sync). And you can drag a bunch of files to a "stack" in the dock; we'll see if this is the cure for the cluttered desktop that befuddles so many writers that I know.
Leopard comes standard with all new Macs shipping today, but if you want that new-Mac experience for your existing hardware, it will cost you $129 for the single-user edition or $199 for the five-user Family Pack. You'll also need to spend $79 to get a copy of iLife '08 (also included with new Macs). Leopard works much better if you have a .Mac subscription ($99/year). I also recommend spending $79 for iWork '08 to get Keynote, Apple's superior alternative to PowerPoint. Yes, discounts are available on some of these items, but that's still more than $300 per year to keep your Mac up to date with Apple's latest software and services.
These products are all worth the money if you value having a computer that's fast and easy to use more than you value, say, 100 gallons of gasoline or dinner for four at a really nice restaurant. For me there's no question: I bought them all. But people who are thriftier than I would probably do better to hold off on this update.