It's wholly unnecessary, but this sequel/prequel is frothy fun for those who enjoyed the original's music and upbeat energy. Streep is definitely missed, but she does return for a surprisingly poignant cameo. The flashback story starring James et al. is entertaining enough, and it's a treat to see Christine Baranski and Julie Walters' odd-couple comedy antics carried through in the earlier timeline with Davies and Keenan. The three lads all believably portray younger iterations of Firth, Skarsgard, and Brosnan, with Davies' Sam, in particular, making it clear which of the men Donna truly considered a potential partner. It should also be noted that the younger actors have considerably better voices than the older dads, although, once again, it's the women who do the heavy lifting.
The movie's ABBA-filled soundtrack includes some repeat titles from the original, like "Mamma Mia," "Knowing Me, Knowing You," "Dancing Queen," and "The Name of the Game"; most of the additional songs are lesser-known B side tunes like "Angel Eyes," "Andante, Andante," and "When I Kissed the Teacher." But there's a fun "Waterloo" number that takes place in a Napoleon-themed French restaurant, as well as a perfectly timed rendition of "Fernando" featuring Cher, who cameos as Sophie's celebrity grandmother. It's not an exaggeration to say you wait most of the movie for Cher to show up, but she's worth it.