^ as a disappointed mac painter 12 hater, I can say that brush performance is the only thing I'm not having any particular issues with
(snow leopard here, that's not supported on the current mac hardware, unless you manage to install the expensive server distributive... which is not worth the hassle, given how unstable and overall laggy 12 is.)
there's a known issue with wacom drivers under mountain lion that causes a lot of point/click lag everywhere when driver is running, though, so it will affect other graphics packages you will be using as well.
on the fourth day of glitchmas my painter™ gave to me
four random crashes, three broken brushes, two system hangups & one corrupted workspace