I believe it's more meant to be subliminal than anything else. If you see more people using iPhones, drinking cokes, driving mercedes, etc (wether in real life or in movies), then you tend to associate that with the norm/popularity/good.
I think product placement is the best way to do advertisement, so long as it isn't too blatant and/or affect the story.
A show called "The Glades" was a big Kia advertisement... it was really distracting and annoying