Really good. Probably better after some kind of marathon of the other related movies, but it was entertaining nonetheless. It was nice to see that by the end, the characters were able to fit into their proper roles without too much alteration (meaning, it's good that they didn't switch Iron Man into the "leader" role just because he's played by the overbearing Robert Downey Jr, which is something I'd always feared beforehand; that role is meant for Captain America, and it ends up working really well just the way it is). Also pleasantly surprised that Hawkeye and Black Widow, being the resident "normal human" characters, don't just fade into the background while the shit hits the fan at the end; they both have a good share of suitably badass moments just like everyone else.
Not thrilled with the villain choices though. I always thought Loki was a lame threat to level against an entire superhero team to begin with. But now apparently they're intent on forcing Thanos into things as the series goes on? Thanos was just Marvel's ripoff version of DC's Darkseid to begin with, so I'm not exactly a fan of the character. Darkseid is way better, but now if DC tries to do anything with Darkseid, it'll be labeled a copy by the uninformed masses. Blah.