While much has been made of some new options that lets users go back to the Tiger look-and-feel for some elements, like menu bars, those are minor items. Frankly, I adapted to the new Leopard look quickly, and suspect that most users won't even notice the changed GUI after a few days.
If you are using Leopard, you should install this update. That's especially true if you use Time Machine, the new backup software in Leopard. The 10.5.2 updates fixes some significant problems in that new application.
Note that after you install the 10.5.2 update and reboot, you should run Software Update again. This will install an update to the core Mac OS X graphics engine.
At the same time, Apple also released security updates that apply to Leopard and Tiger. Apple deserves kudos for being so responsive to security issues.