The most impressive special effect in the whole movie is in the first scene. Michael Douglas, who plays Hank Pym, the original Ant-Man, turned 70 during filming, and he looks it. But the first scene is set in 1989, and in that scene he looks at least 25 years younger. (Agent Carter is in the scene as well, having been aged in the other direction to around 70, but a very well-preserved 70.) This isn’t the first time digital de-aging has been used in a film. Patrick Stewart appears as a younger version of himself in one of the X-Men films, but that shot was brief and not entirely convincing. This time you can’t tell it isn’t a much younger Michael Douglas. In fact, he looks even younger than he’s supposed to be. When Douglas saw the final effect he reportedly started joking about making prequels for some of this earlier films.
The new Ant-Man is Paul Rudd, a criminal who robbed a major corporation of huge amount of money and then gave it to the customers they’d ripped off.
Evangeline Lilly (who, as someone else pointed out, just keeps getting better looking; when she hits 100 she's going to be amazing) is Douglas’s daughter and possibly Rudd’s future girlfriend. In the last two Hobbit movies she played an elf strangely attracted to a dwarf, so it makes sense that she might want to date a guy who’s really small.
It’s a fun film with a lot of humor, and my one real complaint is that so much of it is predictable.