Mac OS X “Leopard” (10.5) is due to ship sometime in October 2007. This will be another major upgrade of Mac OS X, one that will no doubt bring useful features (like Time Machine) and enabling features (like 64-bit system libraries) but perhaps more useless junk in the spirt of Dashboard, and certainly a few new bugs.
Should you upgrade? Yes, but I’d recommend waiting three months before risking “production” machines (ones you rely on to get work done). It’s also a good idea to upgrade a non-critical machine first, to see how things go.
Leopard can be pre-ordered at amazon.com, which offers a low price guarantee and free shipping. Some people will want the single-machine license, but those with multiple Macs at home should opt for the family pack version, which licenses five machines.