I enjoyed this, truly. It's a beautiful love story. But it lacks the spark that would make me love it rather than mildly like it.
I really like Cal, he's a good guy. Jim is also very likable and his struggles made my heart ache. He initially struggles with a lot of shame and self-loathing because of his love for Cal, and that was hard for me to watch. It's hard for anyone with a heart to see, I think, but Jim and his internal struggles made me think of someone I know and so it was much more personal for me. So, yeah I liked both main characters.
As for other characters? Sophie was such a little brat at first. Adam was kinda cute, but I've seen cuter kids in books. Mrs. O'Brien was a fantastic lady and I have mad respect for her. For someone living in the 1940s, she's a very open minded person.
The story is told in two parts: flashbacks to the war and in present day. And while I enjoyed the flashbacks at first, at some point they started to interrupt the flow of the story. The pacing in general is just really uneven at parts.
I liked that the ending was happy and I really liked how in the epilogue Sophie found a way to tell Cal that she didn't see anything wrong with their love. However, a few things were unresolved such as Cal's conflict with his parents.
Tl;dr version- In the end, I liked Semper Fi. I didn't love it though and there are so many other great m/m romance books that I would recommend passing up on this one in favor of others.
If you're interested in finding m/m romances to read, here's my list of recommendations:
Fever Pitch by Heidi Cullinan (college-aged characters, contemporary. Music is an important subplot point)
Winter Ball by Amy Lane (adult characters, contemporary. Imo this is a really sweet and heartwarming story)
Willful Machines by Tim Floreen (young adult, sci-fi/dystopia. Bittersweet)
Out of Bounds by A.R Barley (college aged characters, contemporary. Bit melodramatic at times but ultimately a very cute love story).
What they Always Tell Us by Martin Wilson (young adult, contemporary. Romance is more of a subplot, but I still highly recommend it)