When you look at game site forums like Hellgate:London (yes, Hellgate:London HAS in game ads on top of their other rental subscription BS
To be fair, they're context relevant there. The only ads you see are in the subway on the wall or on a billboard in a world where they fit. I'd rather see real ads there than some made up crap that's just trying to be clever, it adds to the game.
That said, in most games I'd rather the ads weren't there at all. The only games where I don't mind them (or even like them) are games where they fit the setting; driving games where the billboards along the road have real products for example. They're not really selling you anything as much as just making the game world seem more realistic though, which is a point I think these ad agencies are really missing. I don't buy the exposure=preference logic they tend to use.