Shin Koihime Eiyuutan is something of an alternate fandisc for Shin Koihime, including characters from the 'extra materials' that have come out over the years and a canon story flow from the original that seems to be based on Kazuto somehow being on all three sides at once during the wars (lol). To be honest, for consistency, Moeshouden was much better, because it uses the Gaishi setting to explain everything, lol.
Having played all three, I can safely say that it is a very satisfying after-story for someone who liked the original Shin Koihime Musou. However, one thing I also noticed was that, for all that it went first, the Shoku version was the weakest of the three by far. Part of it is that in the original, both Go and Gi's paths were far deeper than Shoku's (Shoku=Shu, Go=Wu, Gi=Wei), with Kazuto taking a far more unique role in how things played out. For better or worse, in the Shoku path, he was mostly the voice of rationality behind the idealist throne that was Touka, whereas in Go he served as a husband and one of the tacticians, and in Gi he was given the role of setting up the training of new troops and the implementing of his own idea for policing the capitol. This difference in roles was part of the reason why the stories each differed so wildly.
To be honest, one thing I'm glad they did away with was certain events revolving around Sheren (Sun Ce) and Meirin (if you are familiar with history, you know that sad stuff happens). While the scenes involving those events in the original Shin Koihime were awesome in every way, I still felt a sense of relief that canon had changed that particular aspect.
Most of the events in the three versions are either emotionally touching (for those that read the original material), mildly interesting intellectually (though mostly with Gi and Go), or H-events (lots of them). I was surprised that Yue was sort of pushed to the side in the Shoku disc, as she was one of the top-rating characters on the old character polls, lol. On the other hand, all the Gi and Go characters made appearances and had their own little scenes during the game.
Overall, this is just a treat for those who played Shin Koihime. While the Shoku disc was something of a disappointment, both the Gi and Go discs were interesting enough that I didn't feel cheated for paying for them. I do wish they had put it up on dlsite or Getchu instead of DMM though... fooling DMM's region-locking setup is a serious pain in the butt.
Edit: I recently finished re-playing Shin Koihime Musou (one of the VNs I've been replaying bit by bit over the last five months)... and I'm debating on whether I should do a full post on it, lol.