When I first read Cosmopolis on its release I thought it was okay but not great. After the heft and glory of Underworld, I figured The Body Artist would be his throat clearing and that Cosmopolis would get back to critiquing the world. (Which is not to say I disliked The Body Artist -- I KNEW it would be nothing like Underworld and was okay with that). But I felt that Cosmopolis's statements about money and art were nothing grand.
Then I read it again while preparing to write my MA thesis on DeLillo and discovered that Cosmopolis adds to his overall commentary. Perhaps I just got so deep into his work that I believe the book works better than it does, but I suspect a second read would cause alot of the mediocre reactions I've heard and seen out there to change for the better.
Then again, I still get sad when I see how many people hate Underworld.... so maybe I'm just a DeLillo optimist.