I think this one is better than the Demigod Files, but I'm still not that into these filler books. If you love Percy Jackson and the Heroes of Olympus it's worth checking them out if only because you get a little more of the characters you like.
I skipped the last story completely though. I don't really care what Rick Riordan's son has to add to this universe. Sorry.
The Demigod Diaries was a cute addition to The Heroes of Olympus series and after almost a whole year without him, it reminded me why exactly I like Jason so much. Well, more honestly, it reminded me how I instantly liked him when he was first introduced in The Lost Hero.
The Diary of Luke Castellan
Luke took center stage in the first short story, which I was really looking forward to. I know Luke was a bad guy, but I have to admit that I liked how he was portrayed before his turning evil and all.... Luke and Thalia are traveling together and meet a son of Apollo, Halcyon, who almost gets them killed and foretells Luke's foreboding future and Thalia's well, what do you call a future in which you are turned into a tree? This was actually the highlight of the story for me. Seeing their reactions was great and they were (of course) spot-on. And how can I not mention the Greek fire? Yes, they make Greek fire. All in all, it was enjoyable.
Percy Jackson and the Staff of Hermes
Next we have Percabeth on one of their typical adventures. One of the Greek gods (this time Hermes) interrupts them and sends them on a quest. My favorite aspect of this one was probably Hermes' and Percy's interactions. Hermes has always been one of the more gracious Greek gods without loosing any of his respectability. Percy's narration was wonderful and sarcastic as always, which makes the story fun. Annabeth was....meh. She was Annabeth and, honestly, who expects a celebration for their one month or two month anniversary? *sigh*
Leo Valdez and the Quest for Buford
My favorite story of the four was most definitely the third one which featured Leo, Jason, and Piper. Oh man, I love them so much. Leo brings a serious but sarcastic vibe to the story. Jason is all calm and cool. Piper is the straightforward, no nonsense girl. Together, they set off into the woods of Camp Half-Blood searching for Buford (a walking, temperamental table) and meet some of Dionysus's crazy followers that even the Olympian avoids. In the mean time, they only have a hour before Bunker 9 explodes if they don't find Buford. I really love how the three demigods interact with each other. And, honestly, this one felt the most like the good ol' light-hearted aspects of Riordan's Greek mythology series which was definitely wonderful.
Son of Magic
The fourth and last story by Haley Riordan.... I was most excited for this one. We were finally going to see the goddess Hecate! While the portrayal of Hecate was wonderful and great, the story as a whole was disappointing. The writing was fine but I wish the demigod, Alabaster, had been the narrator. Instead of him narrating, we have a middle-aged man (Dr. Claymore) and I didn't like that. I think it would have been more enjoyable and more informative if Alabaster had narrated and it would have brought the story's focus more on him. As it was, the story was about Dr. Claymore more than Alabaster, which was really disappointing because Alabaster seemed like an interesting and even honorable character. There were a few interesting revelations though (how monsters can smell demigods and who controls the mist) which were probably the highlights of the story.