Ok, I'm just out of a Phase 2 marathon, starting with avengers 1 and ending with AoU, so I'm rather tired, although very glad of the experience, ans I wanted to give my thoughts before I sleep on it. When the credit rolled, at first I didn't know what to think, and it's still a bit confusing to me now, but overall I don't think I liked it that much.
Seeing all the prior movies before might have hurt my apprehension of the movie, but I thought it wasn't as fun as avengers 1, despite the very good jokes here and there, especially when GotG was shown just before and was just pure fun. The "seriousness" note they tried to inject was far outmatched by TWS, didn't care much for the romantic relationship elements, too much focus on them for my taste.
Ultron was...weird. Not what I expected, He was not this driven AI I thought he would be, he was more like a confused mental patient. And they could have make him so much more threatening with Spader's voice, it's a shame.
For now, I'd say Whedon should have kept it simpler, it worked very well for the first one. I'll see how I feel after a good night sleep, but I give it a 6/10