I thought it was pretty good. There are not many movie series this big and we expected to be rewarded for watching them all. I think we got a good payoff. It needed the long running time to fit it all in and still some characters didn't have much to do.
You need a spoiler warning at the top.
Quill does act dumb, but Strange must have something planned given that he was adamant he would not give up the stone and he must have seen something when he looked into the future.
Thanos is a great villain with some depth. He gets to show some emotion, but his plan is just crazy. If you kill off all those people the population will still recover in time.
I definitely wasn't expecting it to be as good as it was and I agree that we got a pretty good payoff, they could have maybe built Thanos a little more in the previous films though because it felt like he just came out of nowhere for the most part.
I hope Strange has something up his sleeve otherwise that choice was just idiotic, Quill though was a complete moron for that, no excuses! Thanos definitely has a flawed plan no arguments there but I believe that he believes it will make things better.
Spoiler warning added!