Few films are made for a set of audience who follow the franchise and are related to the previous installments that came before it. So does that mean the neutral audience that haven't seen previous installments shouldn't watch them? No, they can still watch the film, but they won't be able to comprehend minor plot points present in the film, which is really important in franchise oriented film, such as Avengers.
The film starts with a 'bang'. I liked that opening sequence, where all superheroes put their best efforts and retain their own personalities, such as Captain America telling them to use 'proper language' or Hulk smashing that tank and doing his own thing, without giving a shit about anyone else.lol
Then the actual roller coaster film starts, with the introduction of Ultron and his twisted ideology about 'humanity and human beings'. Ultron, even if is really portrayed pretty well, never scares me. Speaking of which, both Quicksilver and Scarlet Witch got introduced really well in the plot. I liked the way Scarlet Witch is portrayed throughout the film, her development from 'Ironman hater' to 'an avenger', is shown really well, alongside good acting. I wanted Quicksilver to stay for a next film or something, instead he dies in the final act.
Another character I liked was 'Vision', the sequence between him and Ultron is a really splendid one. Another USP of the film is, Black Widow and Hawkeye got some good sequences and a bit of screentime and exposure, rather than being sidekicks. Hawkeye particularly impresses me. Black Widow's dream and her past, on the other hand, could have been explored further. Speaking of which, Hulk and her love story is another thing I didn't like about the film. It isn't handled properly and leaves a really big question, 'was it really needed??".
The war machine, The Falcon, and Nick Fury just do their job, not much is expected from them anyways. There are few continuity problems in the film as well, typically if we relate it to previous Marvel films, such as that 'Helicarrier sequence', it is difficult to figure out, from where Nick Fury pulled that out?? (after 'Winter Soldier')or the entire 'infinity stones' stuff, which will be explained in the next installments, I believe.
There is also that typical 'Marvel Humour' in the film and you might like it, if you are a Marvel fan.
So, film gets 8/10 from my side, it is must watch for Marvel fans, but if you are going to theater with the thoughts that you are going to watch a 'milestone film', maybe you should reconsider.