According to what I've found via a cursory online search, as of 2006 the first fifteen volumes of Nana had sold 34, 500,000 copies. And that was only good enough to make it number four on the list. I'm not arguing sales quantity equals quality, just that certain big shots in the United States seem to be all turned around as to where the "real" comic books are coming from these days. Even with manga in a slump, it's still far and away the pop cultural leader in sequential storytelling.
I'm not saying manga are aesthetically superior. Some books are, some are not. Just that those of us who sneered at them in the past need to re-evaluate from a broader viewpoint. My conclusion is manga fandom is comic book fandom. American books starring characters like Superman and the X-Men are the niche books.
I'll go so far as to say it's time to start talking about Yazawa Ai-- among others-- in the same conversation with the likes of Alan Moore and Jack Kirby. She probably has more fans and there are probably more people in the world who can name characters she's created and tell you what they've done. Again, that probably sounds a lot like "quantity equals quality," but believe me-- in her case, the quality is also there.