We watch very few movies in the theatre, so it takes a lot for us to decide if we will take the whole family out for one.
Incredibles 2 was completely worth it.
I'll try not to give any spoilers, but Jack-Jack steals the show, without overshadowing everything. It easily could have degenerated into being all about him (as other movies may have made that mistake), but he was a vital and important part, but not the driver of the plot. Good villain, good heros, and a "kids save the day!" without it being unbelievable (and without them driving the movie, either.) Minor issues in some of the character regression, but otherwise well done. I'd not say it's as good as the originial, but still one of Pixar's best movies. (Funny note, my wife cried at the short at the beginning.)
In the theatre, my 20 month old is probably about the same age as Jack-Jack represents and also stole the show. Jack-Jack did something funny and she, very loudly laughs, points, and says "BABY!" At which point everyone in the theatre laughed too.