Beware of the Spoilers. I didn't write anything directly here, but there may be hints that lead you to foresee what happens.
I want to start by saying that we were all wrong. Corpse Party -THE ANTHOLOGY- Sachiko's Game of Love - Hysteric Birthday 2U is not a story about Sachiko's birthday. It's a story about a little miracle. So small that it may even pass unnoticed if you are not watching carefully.
First, you should stop thinking about this game as a spin off. I'll be clear about this: The 8th chapter (that, by the way, made me cry like a little baby) is even more important that Book of Shadows and the extra chapters of Blood Covered, all together (yes, including Blood Drive). You may think that I'm exaggerating, but let's simply say that if we ever see a happy end in Corpse Party (that, by the way, I bet we'll see in Blood Drive), it will be thanks to this chapter, and not thanks to the things we saw in Book of Shadows' Blood Drive.
I won't lie. Sachiko is my favorite character (I have a hero complex, and she is clearly the one I want to save the most). That may make my opinions quite subjective, but read me until the end. I learned a lot about Sachiko in this game (obviously, she's the main character), but also about other important characters, like Yoshie; and the world of Corpse Party in general. That allowed me to understand better what is happening, but also left me in a position where I can no longer imagine what will happen next (specially since I'm not sure whether we have two simple time-lines or one quite complex... call me crazy but right now I think it's only one). And I can say, without a doubt, that the truth is stranger that it seems. I will only ask you to remember a few things: What's Sachiko Ever After? How does everyone do it? And what's the right way to do it? If we think about that and forget about the killings inside Heavenly Host, everything becomes quite simple, so maybe that's what we must think about.
The game it's quite enjoyable overall. Specially chapters 7 (with things like the PC98 room, MORISAW, or the great wrong end, "The dream of R'lyeh" (if you don't know what is R'lyeh, go google it right now)) and 8 (for the reasons above). It has a huge amount of information (characters, a time-line, explanations about Book of Shadows and Blood Covered, and even the notes and documents you found in Blood Covered). There isn't a single reason to avoid this game (unless you don't like Corpse Party at all).
The whole point of this post is: don't underestimate this game. It may look silly and unimportant (or even uninteresting), but it isn't, and if you want to understand what is happening, then you must take this game as it is. I'm sure everyone will enjoy it, and not only for the comedy.
I'll end quoting Yoshie's last words in the game: "The miracle... couldn't happen"... Fortunately for us and the people trapped in Heavenly Host, she is wrong. The miracle IS happening.
P.D.: The more I read this entry, the more I think I'm talking about Higurashi and Umineko. "If everyone believes, a miracle will happen". "Without love, the truth can not be seen". Maybe it's not wrong to use those in this story... Maybe I'm just too sleepy.
Now, beware of the gallery. It contains some of the images that I though were most interesting (you may think it's quite centered around Sachiko, but that's just your imagination). Most of them are from chapter 8.
Message from Dom: Removed the gallery as it contained pictures that weren't there or will be replaced in the future.