In a marathon setting, Iron Man 2 is pretty good. It's shallow, but it's got some great action sequences, fun villains, and lays lots of groundwork for future movies. Like, when you're getting 1 Marvel movie a year, it kinda feels like filler, but when you're plowing through the whole thing years after the fact it's a fun ride.
That's true of all Marvel movies, though. Even at their worst - and there's at least half a dozen that I'd put in the "worst" category - they're still very watchable, competently made action movies.