Initial impressions:
GUI is faster, somewhere in the 20-30% range. Noticeable. Still room to grow, but I bet it's almost glass on the G5s.
Exposé is every bit as cool as I had hoped. Best GUI feature I've ever seen.
Everything just feels and looks right. It looks and reacts like a more finished OS than Jaguar.
Preview destroys Reader. It is super fast on DP 867, can't imagine how it's like on a DP G5. Reader wishes it could be Preview.
Font Book is giving me fits. Poor documentation doesn't help. Thousands of fonts bring it to its knees. Installed fonts don't always update visually, so you have to quit and relaunch FB. By default drag and drop install turns the fonts on (not a good default). And default pref is to move the source font instead of copying (also not a good default). Major headache. Should be worth it in the end, cuz it's great once the fonts are in there. If they ever get auto-activation, this thing will be unbdeatable. I have every OS X Font Manager and have used all of them. FontBook is the best (though SuitCase still seems to be the only one that shows multiple font cases at once for comparison. All font managers should do that).
Apps definitely launch faster. Apple's apps launch very fast. The big apps (Photoshop) launch about 20-30% faster. Don't know how much of that is related to clean install.
Like the new smoother look, tho I'm starting to join the anti-brushed metal bandwagon. I'll tolerate it one more round, then it's time to move on, Apple.
--------
"Discoveries"
I love the Finder-level archiving to zip files. Very nice touch. Goodbye Stuffit.
The Folder Actions are cool, hadn't tried them before. Still not sure I'll ever actually use them, but it's neat to have.
Alt-Tab app switching puts a nice big menu in the middle of the screen. I like that.
Selecting Finder files puts a box around them for easier visual feedback as to what is selected. Nice.
Finder windows show summary info as to that is in the window. I just wish it would add File size info as well.
Why on earth did they put Login Items as a Tab in Accounts Preferences? That was a terrible idea. Took me forever to find that.
My printers are ready to go this time, with Jaguar I had to manually install drivers for two out of three printers.