I don't have it so if you wish to ignore my advice by all means do but how much money do you have left and are you waiting for anything in particular?
After the rather mediocre reaction it seemed to have gotten upon release (that I had seen, perhaps I'm wrong) I'd say that if you are short on funds (less than 20) and are waiting on any high cost games (ARMA 2, The Witcher 2, etc.) or multiple smaller games (Just Cause 2, Counterstrike, Hard Reset, etc.) then I would advise holding off at a 66% discount.
You could then hope that it goes 75% off in Winter if you don't need to play it urgently (if you have Left 4 Dead to keep you and your friend occupied then even better) and at that stage you may not be questioning whether or not to buy it.