Watched it yesterday and thoroughly loved it. I'll go so far to say that it was better than the first in terms of story, character development, and narrative. The jokes hit so hard and so fast that its easy to miss.
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Finally, no need to wait until the VERY end of the credits because there's no post-credit scene. Just a blurb about the support of the movie goes on to help the 15,000 jobs involved in production, which was nice. I'd say leave after the mid-credit scene.
Overall, I highly recommend this film to any Marvel movie/comic fan. Do you have to see the first one to understand the second one? No, BUT the first one is also good so see it anyway.
5 stars out of 5.
Dubstep.