We are the customers, and if you ask the customers (who are, I'm told, always right), Curt Swan is one of the most appreciated comic book artists in history.
I don't know about that.
Sure, those of us who love Curt love him intensely, but in terms of sheer numbers, how many of us are there? Is there a "Curt Swan Collector" magazine out there to compete with the Kirby Collector? Do Curt's original pages fetch anywhere near the figures of a Neal Adams or Jim Lee page? When you consider Curt's contributions, he really is underappreciated.
I don't think it's so much that Curt doesn't get his props as an artist, I think it's just that his talent is totally taken for granted. Consider: There's so darn much of his work that it doesn't have the appeal of rarity that Adams' or Steranko's does. It's in the style of classic illustration, and thus not as flashy or idiosyncratic as Ditko or Kirby. It was done for the supposedly staid DC so it doesn't have the air of boldness or innovation people associate with early Marvel. Few people would dare suggest that Curt wasn't a great artist, but he was not a ground-breaking artist and he worked in a time where people were breaking ground all the time. Plus he'd been around for so long everyone just figured he'd always be there, so instead they focused on the "next big thing," whomever that might have been that month. And then, guess what? One day he wasn't around anymore and it was too late to give him his day in the sun.
I remember reading articles in the 80s where creators like George Perez would heap endless praise on Curt Swan, but I wasn't hearing it from fans. And when you see a guy like Todd McFarlane (whose talent is limited at best) making gazillions of dollars, or a guy like Alex Ross coming from nowhere and getting books written about him overnight, you have to readjust your notions of what is an isn't a "fair" amount of attention. By those standards, Swan didn't get nearly the appreciation he deserved.