The potential reward of product placement is that the product in question will get a great deal of exposure. In today's world, regular advertisments are becoming somewhat less effective. This is particularly true as more people come to won DVRs and can simply and easily skip commercials. Product placement offers a solution to this. It allows the product to be displayed in such a way that it cannot be ignored. This makes for a more effective "advertisment" and can lead to higher sales.
There are two main risks. One is that the movie or show will not be popular and the product might come to be associated with a failure. The more important risk, though, is that people will become annoyed as there come to be too many product placements. They may get irritated by the thinly disguised commercials and come to resent the brands that buy the product placements. If this were too happen, the tactic would actually be hurting the companies instead of helping them.