I feel a bit conflicted about Ultron. I really like his humanity, his humor, the fact he makes mistakes, his cynical hatred.
On the other hand, he isn't all that powerful in the end, certainly not needing 9 avengers to beat him. I know it would have been a weird mcguffin, but if he had the aether in his final form, with the red glowing eyes and gaping mouth, he would be an all powerful menace.
In the way they did it in the final act, Vision alone is more powerful than Ultron because he has an infinity gem. If you make Vision that much stronger, then maybe it would have been better to make Vision's allegiance more ambigious till the very end. Have him be an observer and in the end he can decide to grow a cape and destroy his stepdad Ultron.
So yeah.. conclusion: I love Ultron's character, he is a more demented and in ways a more human version of Loki. But for godssake, make him stronger, he'll look like a puppy next to Thanos.