I saw this on DVD and thought it was pretty fun! I mean, not a masterpiece but still fun. The younger actors were all good, Lily James especially. I too wondered why they didn't get one of Stellan Skarsgard's kids to play him since they all seem to be in the biz. But Josh Dylan actually really resembled SS. I might have thought he was related to him! The plot was a lot of fluff but that was okay. The Sam and Donna confrontation when she finds out he's engaged to someone else was surprisingly emotional. I'll chalk that up to the actors. But Meryl swooped in at the end to make me cry with My Love, My Life.
I read a comment that made a fun point that I hadn't noticed: Sam is the only one that Donna sang a song to, instead of him singing one to her (with her joining in). Nice way to show Sam was the one she really fell for.