Wifi was wonky with the release version of yose for me personally (early '11 13"MBP), then was fine again after the first update. (Still fine with the second update btw.
Despite the weak, aging hardware of my laptop, OSX still runs just fine on it. I've got it connected to a thunderbolt display, Safari is smooth and responsive even in fullscreen mode, and I love the new updated UI. The grey of previous OSX versions was dreary as hell, and that damn linen texture was really bugging me whenever I caught a look of it. Now, OSX is white and pretty and looking great. (YMMV, of course...) I'd NEVER change back, short of on pain of death.
Not to mention security updates, which is really becoming a Big Deal these days. While I believe Apple still patches the two prior OSX versions, I still prefer being fully up to date.
So yeah, OSX has never felt or looked better, methinks. If you got, say, a time capsule or other backup device, maybe you'd want to give yose a test spin. That way you could always roll back to the exact state and configuration of your previous OS if you don't like the way it is now. However, maybe just bite the bullet, switch over now and get used to it, because Apple's not (for better or for worse) going to A: go back to the way it was, and B: keep updating old OSXes for very long. A couple years more probably. Then they'll drop support, as usual.
You'll get a couple more features which you'll maybe like out of the bargain (the iOS style notification center's kind of neat for example), and it's free, so why the hell not, right? Heh.