For my own experience, I upgraded two computers without issue, a 2012 27 inch iMac and a 2011 11 inch MacBook Air.
The installation of 10.9 removes Java, so when you first start Mathematica, the OS prompts you to let it find and install Java for you. Once you do this, everything should be fine.
Judging from the comments on the community discussion, you might want to launch Mathematica with an uncluttered desktop so that you can see the dialog box about installing Java.
On Oct 28, 2013, at 12:40 AM, Raul Martinez <firstname.lastname@example.org> wrote:
> All, > > I use Mathematica 9 on a MacBookPro currently running MacOS X 10.8.5 (Mountain Lion). > > Yesterday, Apple released MacOS X 10.9 (Mavericks). > > Has anyone tried to run Mathematica 9 using Mavericks? If so, did you encounter problems or was it a seamless update? > > Thanks in advance. > > Raul Martinez >