As a huge hip-hop head, I will just add that there are two aspects to evaluating rhymes: How it sounds when delivered by the artist, and how it reads. The best rap succeeds in both domains. Bob Dylan won the Nobel Prize because he succeeds in both domains. In this case, we obviously can only consider the second domain, how it reads. So we should evaluate the results exactly how we would evaluate any other poem (while giving due respect to the conventions and tropes of the form, ie, rap has its own classic metaphors and themes).
