This movie is great. Robert Downey Jr. carries it, obviously, but Terrence Howard is really good, as is Gwyneth Paltrow and a couple of the smaller parts. Jeff Bridges is good, not quite great.
John Favreau is great at directing action, but even better at comedy. This movie wasn't the same old "let's make inside jokes about the comic that fans will think is funny..." It was legitimately hilarious. There was more comedic chemistry between Robert Downey Jr. and his mechanical assistant in one scene of this movie than there was in all of both of the Fantastic Four movies.
The character developments of Pepper (Gwyneth) and Stark (RDJ) were believable and really interesting, thanks to great writing, directing and acting (of course).
The movie got pretty predictable at times, but it was forgiveable, because when what you expected to happen happened, it was even awesomer (TM) than you were expecting.
And yeah there are some technobabble parts where you're like, "I call BS," but I guess I didn't mind that. It's what I've come to expect from superhero movies.