I clicked "install" because I was excited about proper multi-monitor full screen support and then realized I'd likely run into compatibility problems. I've run all my commonly used third-part software at least once (Spyder4Pro software is fine). The only two "compatibility" issues I've had are with the Android emulator (parts of the emulated screen is upside-down -- possibly some display driver problems) and Mail.app (it's crashed on me a few times). CS6 is, surprisingly, fine.
I upgraded two point-steps (10.7 to 10.9) on a MacBook Air and three point steps (10.6 to 10.9) on a MacMini (not fully tested). Took 25 minutes on the Air and didn't time the MacMini.
I'll keep you posted if I run into any problems. I haven't tried working with any big files, but I haven't really noticed much of a performance or battery-life difference.