Honestly, it's been very stable on my iMac since the last DP. The unstable features were Handoff and Continuity (but they've been removed and can't say if they're back), and as it has always been with OS X, the Messages sync with iOS devices.
At this point it would need a good job of GUI standardization though to fix all these quirks.
Also, I installed cDock and the good ol' 3D Dock, because I can't stand that zombie Dock, you know, the one revived from 2006 ?
--
On another note, let's hope these new iWork icons in the Trackpad tutorial gives us a hint that we'll finally see an iWork '15 that truly competes against Office...