I'll start with the positives.
It's witty, the dialogue is sharp given proper weight, the interactions between The Avengers was very well written, and the performances are largely fantastic (particularly from Downey, Ruffalo, Hiddleston, and even Scarlett). Some of the action scenes are incredibly well choreographed, and Hulk was really enjoyable. Is it an entertaining film? Yes, for the most part. But it's probably the first record-breaker that's completely hollow. There was absolutely no emotional connection to any of the characters (aside from when RDJ sacrificed himself in the end and almost died), and I found myself to be utterly overwhelmed with the non-stop barrage of action. The fortress collapsing took a significant portion of the film, and I found myself getting bored pretty quickly. There weren't many moments to breathe once the action started. It was undeniably well-choreographed, but really overlong and excessive.
B-/B