First things first: The Sagan Diary is not a novel. I wouldn't even call it a story, really. It's more a philosophical dissertation from Jane Sagan's perspective, which winds up feeling self-indulgent and overwrought right from the start. It reminded me of Noctuary by Thomas Ligotti, where I found myself reading several pages and not getting any real sense of what the author was trying to convey. There's no story or development here; it's just a bunch of stream-of-consciousness ramblings about life.
The book starts with a memo from one of the CDF administration talking about how the data they've been retrieving from the cadets as they leave the CDF has been fairly useless now that they're receiving transcripts instead of full downloads, and then goes on to send Jane Sagan's transcript to illustrate that point. How is that a good start to the book? You're telling me that I'm about to step into 100 pages of pointless drivel? My guess is that Scalzi wanted to use that foreword as an ironic statement on what the CDF administration viewed as important in regards to what normal people view as important. Hell, maybe it was a warning to readers, too, to let them know this was something far outside what he normally writes. Either way, it should have warned me away from it.
With the other books in the series, even when I found them to be lacking in some ways, I could still recommend the books because they were gripping stories with vivid characters and engaging plots. This one, though, should be avoided completely, namely because it isn't even a story. I finished it because it was short, but if this had been a novel-length book, I would have dropped it 25 pages into it. I like Scalzi, and I will keep reading his other novels, but this one stands so far outside his normal work as to be written by someone else. Completionists might be tempted to read it, but as a fellow completionist, let me reiterate: THIS IS NOT WORTH YOUR TIME.