I like it, especially if it puts off prices. I didn't mind the GuitarCenter Ad in Guitar Hero III. In fact, it made it more realistic. Now, If Sonic starts wearing Nike's, and Samus is covered in ads like a racecar driver, THEN we would have a problem.
Oh, and I think I was part of that thing with the demos. I saw a bunch of Kellogs, Got Milk?, Coca-cola, and other ads in a Spiderman PC Demo (By Activision)... I didn't mind. They were mostly billboards.