I've read Understanding before, years ago, and I was astounded. Not because of what McCloud says in it, but rather because I'd found it in my high school library. My shock was profound; Gilford High, bastion of suburban "normality" and regimented schooling, had this epic dissertation on comics. Of course, they didn't have any actual comics along with it so I could examine them in this new light, which raises some serious questions. Why is it that a text on comics is respectable enough for the library, but not the comics it discusses, when the text itself is told as a comic? I can only guess that the answer is something that would probably depress and infuriate me.
Regardless, I've enjoyed this chance to revisit the text that helped me into my current phase as a comics reader. McCloud's work examines comics by means of storytelling, art, and psychology, with their connection to the work of "sequential art" is the overhead concept which only needs refreshing every so often.
However, while I was entranced by the book when I first picked it up, this time I found I knew enough about the craft to occasionally yearn for more specificity than McCloud gives. As I begin to give serious thought to writing actual scripts and submitting them to companies, I find that Understanding is less about what it takes to create comics, and more about what comics are once they've been created. Then again, perhaps this generality is akin to McCloud's point about icons being more applicable; maybe he's sacrificed specificity to reach the widest possible audience.