That will depend of where they put the ads .. if they put them in the right place in the right game .. it will be alright & fine by me .. & like what you said it could make the game even more realstic .
but if they used it in wrong places throught the game like loading or intro screens that will be a disaster . imagine some companies ask for longer loading screen for there ads ! heh
or ads like " Quake IV powered by Pepsi" or " Stalker .. brought to you by Hardeez"