Technically this is the 4th beta of Yosemite, the earlier beta releases going to developers only, but the public will be able to grab the latest beta on Thursday.
Macworld has a fantastic FAQ regarding the public Yosemite beta release that everyone should read if you're interested in obtaining this preview.
The key point in the FAQ is...wait for it... this is a beta of an OS. Ideally you should try this on a secondary machine and not your the main Mac. As Macworld states it:
More important, we strongly recommend against installing the Yosemite beta on your primary Mac. Ideally, you’ll want a separate Mac you can dedicate to running the beta, so that if you have any serious problems, you won’t be stuck without a working Mac—and you can erase the test Mac’s drive and start over if things go completely south.You will encounter bugs and problems in the OS, Apple's apps, and other apps. The whole point of a beta period is to find and fix problems before the general release so don't be surprised if you find some snafus.
I've also personally noticed that not only does OS X get more stable but it gets faster as beta updates continue towards the general release. So don't be surprised if the Yosemite beta is a bit slower than what you expected.
Curio and Yosemite
As detailed in this forum post, I've been getting Curio 9 going with Yosemite since the initial beta release in the spring and it's looking very good.
Earlier versions of Curio prior to version 9 are not supported on Yosemite. For example, while Curio 8 runs on Lion (10.7), Mountain Lion (10.8), and Mavericks (10.9), it does not run on Yosemite (10.10).
Curio 9's codebase has been completely updated to work with both Mavericks and Yosemite. In fact it requires Mavericks at a minimum and the code has been cleaned up and modernized considerably. On top of all of those updates, getting Curio 9 going with Yosemite required a number of additional code modifications, and this is an ongoing process as each beta release seems to require more code tweaks within Curio 9. Yosemite is definitely a major OS X release.
Beyond the internal changes, the Curio 9 user interface has also been updated to look more natural especially within Yosemite's new environment.
Please see the Curio 9 release notes for all the details, including many, many screenshots.
While working with Curio 9 and the Yosemite public beta, please send feedback and comments to Zengobi either via the forum thread or our support email.