What I love about the movie is that it walks a fine line between plausability and farfetchedness. It has the right amount smartassery (in dialogue) and action sequences to keep the fanboys satisfied while enticing the non-followers of the source material. I dig the fact that it's not campy at all, which is a common trap for superhero movies. It's a given that RDJ totally owned his role as the decadent and likeable asshole Tony Stark, but I appreciate that they didn't overplay his alcoholic side (Tony, not Robert).
In spite of the outlandish premise that could spell problems for a live action film adaptation, there is a realistic temperance. One such example is during the scene where Stark does a weather and air traffic control check before doing a test flight. His AI butler Jarvis warns him that there are "terabytes" of calculations needed to do an actual flight, but Stark/Downey Jr. stays true to character by skipping the advisory and blasting off into the sky.
The verdict: go see it if you haven't already, nuff said :)