The fandisc for the entire series actually brings all the characters - plus a new cast of one protagonist and three heroines - together, including extensive after-stories for both the first and second generation of characters (Gakuen being the second generation and the others the first). The new cast is based in an era fifty years after the events in the second generation, when most of the characters are elderly or deceased (with the exceptions of near-immortals like Suiku). The three heroines are the endlessly greedy baphomet (demon) Meti, the shy chimera tapir-girl Lettuce, and the dryad (whose name I didn't bother to remember).
Overall, like most of the entries in this series, the standout aspects are the writing quality and the comedic character interactions. It is especially hilarious to see the casts of all the VNs (including the nukige) from the first generation interacting with one another to bring about the setting of Gakuen. Since the main story doesn't change at all (except for h-scenes) depending on which heroine you pick for the new protagonist, I went with Meti... and didn't regret it. I've always liked weird/insane heroines, and she definitely fits the bill.
That said, the story in this VN is structured really weirdly, due to the fact that the protagonist and his friends are basically looking into the past and interfering with it... and if you aren't paying attention, it can be easy to miss important details. However, that is the mark of a well-written story in any case, as a detailed story usually means a good one, though not always.
There is one VN in the series I haven't touched, other than the Alraune nukige... the mermaid one, but that is supposedly based after the events in all these games (though I only got that through some vague hints) so I am not all that focused on it (in total the games so far have covered over seventy years of history, after all).
To be honest, I never expected to see this much detail out of what is essentially a low-cost series created by a company that originally produced only nukige. From a visual perspective, the VNs aren't excessively impressive, and the backgrounds are reused in each game where appropriate, for instance. Nonetheless, it is an impressive series of games purely from the perspective of writing, and if you don't mind feeling like you want to puke at some of the H scenes... this is a great choice for an experienced reader who is bored with normal romance and moege romance.