Spoiler-free review after the SD screening this past Saturday.
Witty, well acted, very well paced, and some mind-blowing action, particularly the final battle scene. Simply put, BY FAR the best in the MCU, and a slight edge over TDK IMHO as best CBM. TDK was a more polished film overall as I loved it as a crime drama more so than a CBM, but I'd watch Avengers 10x more often as it's THAT much more entertaining. Avengers = comic book pr0n on silver screen, best way to describe it. I haven't had that much fun in a movie since perhaps the original Star Wars trilogy, absolutely no comparison to the first Transformers in terms of popcorn fun.
For reference on my taste, my new order of preference for the MCU - Avengers (by long shot), IM (good stretch over the remaining films), then Thor, TFA, Hulk, and at the very bottom of the barrel IM2.
My gf who is a not a comic book geek by any stretch also loved it and wants to see it again (we have tix for El Capitan May 3 midnight screening), and she NEVER wants to see any movie twice, even her own favorites. She drove with me from LA to SD to get in line at 6am, waited with me for 4 hours in 50+ degree weather (that's freezing for her lol), and she still loved it... that speaks volumes for a casual viewer.