The new OS has a whole bunch of new features, including some whizzy but (in some opinions) questionable new interface changes, but I think the best part is the new Time Machine automatic backup application. It requires a separate hard drive volume of equal or greater size to your main one, but it at last makes simple the process of backing up and restoring files. Finally! (I wrote that Apple should do this a year before the first version of Mac OS X came out, and Windows has had something similar for years, though I know few who use it or know it's there.)
However, if you're running a mission-critical production system, as always, I'd recommend you wait to make sure that all your gear and software is compatible with the new release, make a full backup in case anything goes wrong, maybe test out the update on a non-critical Mac first, and make any updates to your main machine when you have time to spare for troubleshooting. This is Apple's first major OS update in two and a half years, and some major changes have taken place under the hood.
You'll need at least an 867 MHz G4 or better processor (the iMac DV in my kitchen is stuck with 10.4 Tiger), a whole whack of RAM (Apple says 512 MB, but I'd recommend at least 1 GB), and 9 GB or more of free hard drive space to install Leopard. And obviously, the newer, faster, and more tricked-out a Mac you have, the better it will perform. Generally, Apple has kept performance on older Macs pretty good with new OS releases, so I'll be trying it out on my eMac too—on some occasions new OS releases have even been faster than previous ones.
Often the major limiting factor isn't your processor or storage, but how powerful a graphics chip your Mac includes, because a lot of the new features use the graphics GPU for screen eye candy like Cover Flow and other animations.
Prices are much the same as before: about $130 for a single license, $200 for a family five-license pack, and $116 for the education rate (a bit higher than before). The cost is the same in U.S. and Canadian dollars, although even at that it's now cheaper by a few bucks for us Canadians to pick up the American version. I know, we can't shut up about that. If you buy a Mac with 10.4 Tiger on it between October 1 and the end of the year, you can get a cheap upgrade to Leopard for $10.