In my attempt to rewatch all the MCU-movies chronologically I began with the first Iron Man. To cut to the chase: it's still good. A couple of things astonished me, though:
First of all, a common mistake of the later Marvel-movies isn't made here. There is actually some sort of character development visible. Tony ends the movie as a totally different person. Of course, in an origin story it's easier to have your protagonist change but let's compare for a second: Is Ant-man really a changed man at the end of the movie? What about Star Lord? What about Thor? Black Widow? Hawkeye?
Then again, you have to give it to the director that he really made a well-paced, well-balanced movie. There is action, humour, drama, world establishing, all in great balance. The final fight isn't stretched too long and therefore doesn't get boring. You can't underestimate this in our age of 45 mins battles at the end of movies.
During my re-watch I also noticed how 'down to earth' this movie is if you compare it to some of the more recent MCU-entries. It just looks more real than them. It's set in a world that at least COULD be real which makes us care more for the characters and their actions. More explosions and obvious cgi-effects don't improve a movie. So the movie doesn't feel as bloated as some other superhero flicks.
Finally, Iron Man is still fun to watch and not just a vehicle to set up a franchise. If you've spent the last couple of years on the moon and haven't seen any of the MCU-movies, give this a try.