This build is very very quick on my machine. Responses to user interface elements are instant and windows (finally) resize smoothly and quickly. Some apps seem to take advantage of whatever they're using to make window resizing smooth while others are using the old method. The most dramatic display of this new resizing ability is in Preview. I can open a 4MP image in Preview, have it "resize to fit window" and scale the window up and down from a tiny pic to full screen, the whole while the system is scaling the image and the window with ZERO jitters. Very impressive. I can even have a portion of the image selected (with translucency, of course) and the selection will scale glassy smooth right along with the window and image while resizing.
Spotlight is VERY fast, virtually instant. It finds some pretty impressive stuff that I had even forgot was on my HDD. Safari renders pages much, much faster. There's nice little details throughout the OS too. DVD Player will keep playing a movie even if minimized to dock. System Profiler is far more thorough in its detail of hardware and software. Mail looks great but is very very buggy. Well for that matter, the whole OS is still really buggy, but we're a long way off from GM release, of course.
Overall, I'm extremely excited about this release. There are so many thoughtful details and such a dramatic UI "snappiness" improvement that I'm just counting the days until release. I'd say the UI improvement is on par with going from 10.1.5 to 10.3.5. Tiger is WELL WORTH $129 and I'm looking forward to forking over my cash.
-Drew