In my experience testing it on my Mac mini (Oct 2012), 10.9 DP8 is pretty reliable. There are very few bugs that I encounter on a regular basis. There are a couple of seemingly-Magic-Mouse-touch-scrolling-related bugs in the App Store and Safari that I have reported. Beyond that, I have found a couple of apps with have Mavericks-specific issues (e.g. Calibre crashes when trying to convert ebooks and Plex Media Center crashes or hangs if it is launched with the menu bar focus on a secondary display). And on the cosmetic front, most apps that manage their own full-screen views (rather than use the official OS X full-screen API) have issues with the menu bar appearing on top of their full-screen views on secondary displays (because there previously hasn't been a menu bar on secondary displays).
But the multi-monitor support is so much better that I installed on my primary machine's primary partition a few DP releases ago. (I tested it on a separate partition for the first few releases.) And this weekend, I finally wrote over the clone I had made of my primary partition before I upgraded it from ML. At this point, there was no chance I would return to that several week old backup.
Because I have a desktop with 16GB RAM, I can't speak to the impact of power saving and RAM efficiency improvements in Mavericks, but those sound as if they would be worthwhile on an Air.
John