This NASA video puts some perspective on the difficulty of the Rosetta mission. Since the space age began Space agencies has succeeded to land on only six bodies. Venus Mars, the Moon, Titan and Astroids 433 Eros ad Itokawa. If all goes well Rosetta and her lander Philae will add a seventh member to the list comet 67P/Churyumov–Gerasimenko.
How hard is this landing? Watch the video.