When it was first announced, AdSense for feeds generated a lot of controversy, particularly in the blogging community. Many were dismayed to hear that ads might start showing up in feeds, although it shouldn't have been a surprise to anyone—the industry as a whole has been trying to figure out how to monetize syndicated content for a while now.
The question remains about whether or not AdSense for feeds will prove as successful as AdSense for content. Early adopters of the program have reported mixed results and some have removed ads from their feeds as a result of either negative feedback from their readership or negligible earnings from the advertisements. Highly read blogs might benefit from ads in the same way that sites with lots of traffic can make substantially more from advertising that sites with little traffic. The jury's still out on this program.