Once you've committed to an application, weighing the benefits of each new software upgrade becomes an inevitable part of the process.
One issue to consider with software upgrades is to determine the resources the new application will demand. Software "bloat" can drag your once perky system down to a crawl. It's not unusual to need a RAM or even processor upgrade in order to run the latest version of piece of software. And in some cases, the "upgrade" can represent a step backwards. (Read the excellent Videoguys for their take on upgrading to Windows Vista.)
I'm happy to report that upgrading to Premiere Pro CS3 is exactly that - an upgrade. Of course there is an improved feature set for the application. For instance, a very good Time Remapping feature and the addition of H.264 codecs. The upgrade also includes Adobe Encore CS3 for DVD authoring and Adobe OnLocation, a direct to disk recorder. A complete overview of new and improved CS3 features can be found here.
However, the genuine surprise was to see that the application ran faster and in a more stable fashion than Premiere Pro 1.5 on the exact same machine. Nice job, Adobe.