Iron Man 2 is no doubt a fun film with the effortless charisma of Robert Downey Jr, the quirkiness of Mickey Rourke's Whiplash and the great dances that Sam Rockwell brings to stage as Justin Hammer. It also recasts Rhodey as Don Cheadle who puts on such a great performance as the 'best buddy' that Tony needs.
Where this movie falls short is the plot. There are so many things going on throughout the entire movie because the main plot line isn't strong enough to carry a two hour movie. We get introduced to Scarlett Johansson's Black Widow, who admittedly is cool but is just there so that her introduction in Avengers doesn't feel so out of the blue. We get a dancing dork weapons dealer, car races, making a new element and getting some great, and cynical Nick Fury.
But none of that is crucial to the main plot with Whiplash except for Justin Hammer. This makes the movie feel like it doesn't really have stakes. Whiplash shows up, causes havoc, then Tony thinks he's dead for half the movie only for him to come back in the final battle. It's not that interesting of a plot to begin with, but then all tension is taken out of it and it falls short.
Overall I think the only reason this movie holds up is because there is an incredible cast, but in terms of coherent story, it doesn't work so well.