This movie has a bit of something for everyone. From the well-placed comedic relief throughout the movie (most of which is Downey's having complete fun with his character) to the rip-roaring explosive action brought by the larger-than-life hero. They spend a good portion of the movie introducing both personalities of Tony Stark, the carefree billionaire, and the engineering genius. Downey not only manages to play both these sides to a tee, he the steps it up an entire notch in the latter half of the movie, when we see the actual Iron Man character come to life.
Throughout this, Gwyneth Paltrow interjects some great character interaction between "Pepper" Potts and Stark, both in friendly and business banter, and a romantic undertone. Downey and Paltrow both play their respective characters well off of each other.
As to the action, we've all seen CGI improving over the last few years, allowing some of our greatest comic book heroes to come convincingly onto the big screen. Iron Man is no different, and in fact, seems to improve on the digital effects to add even more to the character in full digital renderings. However, even when Downey himself is actually donning a suit for the camera, it looks so incredibly similar to the CGI rendering that you could almost believe such a technological wonder is actually possible.
For the comic book buffs that may have seen the animated movie that came out a couple years ago, you will probably see much of the same thing for the first hour. You will not be bored with it, though, the fresh take with current events and settings of conflict make subtle but important changes, as well as the captures of characters live actors can do that cartoon animations can never provide.
As was mentioned, be sure to stick around after the credits. It doesn't just spell sequel... it opens the door for something that is possibly even better.