Helen and I went to see Avengers Assemble last night: neither of us especially well versed in the previous films leading up to this. Did you need to be? Not really: H managed well enough with some handy muttered explanations of the couple sat next to her, and I had enough geek factor to fill in any gaps.
Of course, its (just) a movie and nothing is ever perfect; but Whedon's classic response aside, this is pretty much a fan-geek's dream come true.
There are plenty of delightful set-ups as the characters come on board, but by the end the film is beyond its stride: it is taking colossal steps over everything and throwing an exceptional amount of wit into the dialogue and gestures as it goes. Oh there is LOADS of action. Plus if you don't come out chuckling at the line "Puny God", you obviously went to see another film in error.
As ever, trailers can be a tad revealing, but if you haven't already seen the film, I'd urge you to go. For the spectacle, for the action and for the sparkling Whedon wit. Brilliant!