I'm writing this post using Yosemite Public Beta and wanted to share my experience with getting the early version installed and how it compares to past versions of OS X for me.
After a troublesome start, where installation of Yosemite never succeeded properly on my older 2007 MacBook Pro with an external FireWire drive, I finally got it installed onto a 2012 MacBook Pro. On the 2007 machine, I had opted to install onto a new partition on the external drive, but this led to the machine hanging 1/3 in to the boot. Several attempts at fixing this failed. Eventually, the installer did run, but then the next boot led to the same hang.
Furthermore, even though I was installing on to an external drive, the bootloader on my main internal drive had been taken over by the Yosemite installer, so I was no longer able to boot into Mavericks properly. I was able to boot up from a spare boot partition that I had formerly created. Reinstalling Mavericks from the Rescue Disk solved this problem.
Installation on the 2012 machine succeeeded on the first attempt since I had opted to wipe the internal drive and install Yosemite directly.
Conclusion: Installation of Yosemite was a poor and time-consuming experience. I fully expected Apple to have refined this process enough to ensure that it was smooth and as quick as possible. However, it was far less rewarding than past OS X installations and upgrades.
On this non-retina display, I'm finding the user interface to be less than impressive. There are four main problems (for me) that separate it from past versions of OS X:
Conclusion: The interface is less refined and impressive than past versions of OS X. Overall, it's still a very mature and unified interface, but work is needed to clean up the small details that have always separated OS X from other operating systems. I truly hope that these are just beta problems and will be resolved by the official launch of Yosemite.
The performance of Yosemite Public Beta is unnoticably different from Mavericks, with the expectation that it's ever-so-slightly slower due to debugging logic running during its operation. If this assumption is correct, then the deactivation of this debugging code should boost performance slightly.
I am not impressed by Yosemite. My gut feeling was that Apple developers were heavily focusing on changing the interface for the sake of change. Improvements to OS X's interface have always been incremental, building on the strengths of the previous version. Yosemite throws all of that away and starts over, transitioning the interface elements to a more muted, less refined appearance. I'm sure the interface looks way better on a Retina display, and if that were the case, and Apple doesn't address these issues on non-Retina displays, I'll be very disappointed. There's still a massive userbase running traditional displays, and they should be getting the same amazing experience with Yosemite as those with the newer display tech.
I have focused mainly on the user interface in this report because that's the most stark change for me. I was concerned about how the updated Spotlight and Notification Center interfaces operated, but surprisingly, they have been non-issues for me.