Some users—myself included—are experiencing issues with multiple monitors following the upgrade to OS X 10.9. From what I've been reading, these range from minor nuisances to serious irritations, depending on the types of video connectors and perhaps some other hardware characteristics.
My own problems fall within the former category: application windows that open on a display other than the one where I want them to be and subsequently fail to stick after a reboot. But if you use more than a single monitor, you may want to check the latest published reports before committing to the 10.9 upgrade. (I won't provide sample URLs here, since new ones keep appearing; a Google or Bing search should provide what you need to know.)