I'm finding it tough going on a 2011 iMac 3.1 with a radeon 6970m 1gb.
to start with i had it on high and it was beautiful but frame rates weren't great, dropping it into medium it looked similar to far cry 3 and seemed okay, but any time theres a fight (i.e. most of the game) i end up playing it on low, which makes it look like far cry 2. and the reason i didnt end up playing far cry 2was the graphics were rubbish.
EDIT: ive persevered, and i take it back, on medium settings it plays well, the game is very similar to far cry 3 but with some nice additions like when you go to the hymalayas in the snow.