I've seen it twice in three days, when it came out.
Going again this week.
I loved it!
I had read people found it boring and took a while to get really started.
But I really loved all the individual stories.
So I agree, it all felt it was necessary, and didn't feel like they were just trying to fill time.
It has very funny bits, and then other moments tears were rolling down my cheeks.
I've been watching every movie (multiple times) since Iron Man 1 came out.
I guess I felt more emotional also because it's the 'end' of that particular phase that I was part of for 10/11 years.
You really start to love the characters.
And, both times, it did not feel like 3 hours at all.
First time I looked at the time and was like 'what, 1,5 hours already??'