I've been using it off and on all afternoon and every time I open the screen I'm like, "Wow this looks really nice." It's subtle, mostly, but still better in my opinion. I like the new flatter look and the Helvetica Neue font in the menu bars is really nice as well. I think it looks very "Pro" so to speak, especially compared to, say, Windows 8.1 (which is on my desktop PC--mostly for games.) So far, my programs and apps work and that is good--I had many issues with the Mavericks beta early on. Both my networked printers work as well.
Sadly, I can't get a lot of the continuity stuff to work, but I think that is because it's beta and I'm still using an iPhone5, not a 5s. I get a bluetooth error when trying to connect it to the Air.
So far, I do not like the strange blue-colored folders in finder, and I'm not a huge fan of the Yosemite background picture... a bit too yellow for me. (JMO)
I am looking forward to iCloud Drive (basically bringing back iDisk which I miss terribly) and the answering my phone and hotspot features. So far this is pretty smooth for a first beta release.