1. This is the second-stupidest of all the Marvel films (Thor is still the stupidest).
2. To its credit, the movie knows it can't be taken seriously, and it tries to have fun with itself for awhile. So it's not a complete disaster.
3. It follows one of the usual Marvel superhero formulas (a super billionaire corporation makes a scientific breakthrough and turns it into a super weapon. It ends up in the hands of a regular schmuck, and bad guys try to get the technology). Which means there is absolutely zero suspense to the story, because we've seen it in so many other Marvel movies.
4. The "scientific breakthrough" is a chemical that shrinks the distances between atoms of things but leaves the original power the same - whatever that means. This chemical is put into a suit that not only shrinks the suit, but anybody in the suit. OK, I admit, I'm actually not sure what the idea is, but I don't think I'm alone - the movie doesn't really want you to pay attention to how or why it works.
5. This movie is a mix of superhero movie and heist caper. The heist caper part (which was the first half of the movie) was better than the superhero part.
6. There are several scenes and conversations in the movie that let you know where it fits in the larger Marvel timeline.
7. This movie features another clunky Disney family drama, another smart-ass female sidekick/romantic interest, and some racial stereotypes. In addition, true to Marvel's worldview, lots of things can get destroyed with no actual consequences, human or otherwise (well, unless you carelessly shoot your own helicopter pilot).
8. The acting is pretty good, all things considered. Paul Rudd is a good choice to play such a ludicrous figure; his tongue-in-cheek charm is the right approach for the ex-convict who rides flying ants.
9. The movie starts off being stupid, but kind of clever, so it's actually fun. However, by the time we get to the last 30 minutes of the movie, it has lost its cleverness. It's just stupid.