Now, normally I don't expect a comic book movie to be anything but fun. I don't go into the theatre expecting excellent acting and a well rounded plot.
But that is why Iron Man is so special. Not only was the writing true to the characters, it was a very good movie in its own right.
I'm normally harsh on comic book movies because of how much of a fan I am. However, Iron Man delivers on every one of my expectations. The acting is excellent, the SFX are mindblowingly cool, and the plot is a mix of corporate intrigue and balls to the wall action.
Also, the ending - after the credits - is just pure nerd candy. That gets extra points.