Movies based on comic books are big business, as you can see from this list of the ten highest-grossing films in the United States based on comic books (as of August 2011).
1 The Dark Knight (2008) $533,345,358
2 Spider-Man (2002) $403,706,375
3 Spider-Man 2 (2004) $373,585,825
4 Spider-Man 3 (2007) $336,530,303
5 Iron Man (2008) $318,412,101
6 Iron Man 2 (2010) $312,433,331
7 Batman (1989) $251,188,924
8 Men in Black (1997) $250,690,539
9 X-Men: The Last Stand (2006) $234,362,462
10 X2: X-Men United (2003) $214,949,694