Well, it's not THAT kind of a twist. It's just a plot twist. It's fun. You'll never experience it again, but you can still appreciate what you felt.
I don't see how it's a gimmick, as it's played into the story and Killian's plan. Kill the President, grab Trevor, bingo. He's famous. He's the biggest. He'd be bigger then Tony Stark every would. I just think people are mad they were jipped out of a Ben Kingsly villain. Which, could've been fine...but unless he got into an Iron Man suit, I have no idea what the final battle with him could've been.
I don't know what Iron Man did that wasn't already established. But before you tell me, you have to take in account where he is at this moment. He's a man who thought he was the greatest, but realizes he's nothing after seeing Thor, Cap, and The Hulk. He's just a man in a can.
So his priorities are going to be different. Hence his constant tinkering, his over production of suits for anything the universe could possibly throw at him, and his new "powers"...which he took from his little watch device in The Avengers.
He became a different character. He grew. It was nice to see.