This movie has the same problem as 'Iron Man 2' had with Mickey Rourke. We needed more James Spader as Ultron. I'm sure if you calculate the minutes he's in it, he has plenty of time, but I need more. His voice is so smooth yet crackly. I could listen to him talk all day, especially as the child-like Ultron. That character is both brilliant and naive at the same time, which Spader plays very well, given the material.
What I like about this one is that Ultron is an enemy of Tony Stark's own creation. He's not the most interesting bad guy, but the fact that this one is on Tony makes for an interesting dynamic within the Avengers, which all leads up to the lack of trust in Civil War, the third Captain America film.