Already being confirmed that James Spader would portray Ultron in the Avengers: Age of Ultron sequel it's notable to say that Joss Whedon has come out and said that his only choice was James Spader and here is the reason why:
"Spader was my first and only choice, he's got that hypnotic voice that can be eerily calm and compelling," Whedon told Marvel. "But he's also very human and humorous. Ultron is not Hal, Spader can play all of the levels. He's the guy to break the Avengers into pieces." Whedon then apparently let out a maniacal, evil laugh (as he is wont to do).
His voice is haunting it's true and it's going to be interesting how Ultron will work out in the end as a whole being introduced in the movie universe but we will definetly see with more news and confirmations!