Well yes im using it right now, just installed it 2 hours ago. Yay!
To be frank i haven't really seen much difference, only that they are going for a more vibrant and "clean" style. Also being a beta its missing a few things like the new Iphoto/Photoshop Hybrid.
As for gaming performance,i have the Intel hd5000 and i only play WoW at the moment but it feels like i got a small FPS improvement (2-5and up to 10FPS in some areas. Anything changed with OpenGL? or maybe its just me).
EDIT:Apparently its not just me:
Will try to update post when i get deeper into Yosemite :P
Something i just stumbled across randomly:
Chipset Model: Intel HD Graphics 5000
Type: GPU
Bus: Built-In
VRAM (Dynamic, Max): 1536 MB
It seems that they upped the amount of RAM the iGPU can access by an additional 512mb. Is this the reason for my slight performance gain? Ive been roaming around the web checking on this and haven't really found a clear answer... some say yosemite has worse FPS and some report up to 30% performance gain (1gb to 1.5gb hmmm sounds logical on paper
).