In short, Iron Man is a well-written and well planned action movie. The writing and direction of the movie makes the character accessible to every one (not just us comic fans). There's excitement, cool visuals and a lot of really genuinely funny moments that drive this movie.
But the real reason to see this movie is Robert Downey, Jr. There hasn't been an actor yet who's had as much fun being a comic book icon and it really comes through in his performance. Whether you're an Iron Man fan or a fan of Robert Downey, Jr., you'll really love his portrayal of billionaire playboy Tony Stark.
I don't know what else to really say without talking about the plot or secrets or anything, so I'll just say two things: First, just go see it because you shouldn't be disappointed. Second, if you're a fan of Marvel Comics, make sure you stick around until the end of the credits!