Most students are not familiar with Calvin and Hobbes.
I like to have a "political cartoon" day for evaluative argument, but have never gotten the lesson to really take off. The main reason: students are not good, bad actually, at figuring out political cartoons. Even cartoons that may not necessarily be "political," but might contain a political, or social, message like Calvin and Hobbes and Boondocks. We usually have to spend the whole class figuring out what exactly the cartoons are trying to say, rather than actually getting to evaluate them.
Therefore, this exercise might be better at the beginning of the semester, when students are learning to hone their analytical skills in general. Students are used to seeing images, like brands, and understanding immediately what they mean. Figuring out a political cartoon is a mind bending and enlightening process, which encourages them to not take images for granted.