EOS Utility works fine under Mavericks. I should qualify that by saying I haven't upgraded to the released version of OS X 10.9, but I have been running the Appleseed versions of Mavericks in a sandbox partition for a couple of months, no issues with EOS Utility, CS6, DxO Optics Pro (v8 and the new v9). I've also seen forum posts elsewhere confirming that tethered shooting with EOS Utility works under the public release of Mavericks. So...you should be fine to upgrade. Pretty painless if you're coming from Mountain Lion, and free is a nice bonus (and in case you didn't know, iWork and iLife apps are free now, too).