After upgrading to Yosemite it turns out APC is gone, and no longer installable. Yosemite comes with PHP5.5 for which there is no APC.
But the new PHP comes with opcache which you just have to enable by adding the following to your php.ini:
On my site here it basically cut my page generation times in half straight out of the box. If you like some pretty graphics and an interface to manage it (and you run WordPress as well) grab the ‘OPcache Dashboard‘ plugin.