By underrated I think he is not as well known as a comic character. He doesn't get the respect he deserves. Look right now I'm logged on and discussing the character and no one else is online. It is 1 am on a friday night (and Im home cause I got work at 8 am) and no one else is looking at the thread. If you were to go to Battys thread there'd be 25 people viewing, theres 10 on Wolverine's and even 50 or so on Spideys. So that's pretty underrated or atleast not respected enough to me.
Most people know Batman, Superman, some of the Xmen, Blade(cause of the movies), Spidey ofcourse, the Fantastic Four, etc. I think if more people knew of the character and/or read his comics instead of passing by them at the store they'd appreciate him like we do. You see, his competition was plentyful, and if more people knew him or respected him he'd compete better with The Dark Knight at the boxoffice. I wish TDK all the best luck but I love seeing Marvel do better.