Not much I can say at this point thanks to the NDA, but the few details I can give:

The OS itself is beautiful. It's the next logical step in OSX. Everything is sleeker and more modern looking than any previous OS. It'll definitely take some getting used to and I can already predict the angry hordes that will overtake the Apple Support forums.
Many apps (Omnifocus, 1Password, Evernote) don't have updated versions compatible with 10.10
There are a LOT of bugs, as to be expected in any beta. Apps crashing, non-responsive trackpad gestures, settings being randomly reset, etc.
Some of the major new features (Continuity, Handoff) aren't even present, some don't work (family iCloud accounts) and others (iCloud Drive) aren't worth turning on, as they'll make your data completely inaccessible from older versions of OSX and iOS.

I wisely installed it on a separate partition from my main 10.9 Mavericks disc. I can't even imagine the pain anyone who installed it on their main drive is going through. Beta means beta, even from a company as polished as Apple.

October isn't that far off in calendar days, but it's a lifetime of development in software. I've got no doubt once the official release comes out these bugs will be long gone. I personally can't wait.