I'd never thought of a good reason to use the comments section in the info section of iTunes, until I was looking up lyrics online. Since the info in the comments is saved as part of the file, it's a good place to keep your lyrics -- when you move the MP3 to another Mac, you'll have the lyrics with it.
The only drawback I have found is that iTunes does not offer a scroll option for the comments box ... time to e-mail Apple.
[robg adds: Keeping lyrics directly with the song is a good idea. Unfortunately, Apple's comment box is basically brain-dead, so you'd have to copy and paste them elsewhere in order to really read them easily. Perhaps the comment box contents could be AppleScripted, though, to automate the extraction process.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040908013634786