Some key features of the upgrade include expanded peripheral support, 2D and QuickTime acceleration on Rage Pro machines, Quartz anti-aliasing in updated Carbon applications, WebDAV improvements, and improved file searching speed.
I haven't had a ton of time to play with it, but I did test the WebDAV improvements. The iDisk is now nearly as quick in the Finder as Goliath WebDAV access. It's close enough that I may stop using Goliath due to the ease of navigating and managing the iDisk from the Finder. In addition, Office with the Service Release 1 update looks great with anti-aliasing in 10.1.5!
I did manage to get a kernel panic during the upgrade; I had switched over to my PC during the installation (I use a USB KVM switch to share one keyboard, mouse, and monitor), and when I switched back, the upgrade had finished and was waiting for a restart ... and then I got the panic. Upon restart, though, all was fine and has been ever since.
I have not yet installed it on either of our Rage Pro machines to see if 2D and QT acceleration make any notable difference in video performance.

