First, I should state that I'm avoiding the after stories for now, simply because I'll want to see them for the first time immediately after replaying the original game, rather than over a year after I originally played it, when my memory for details has faded as much as it has right now. As such, this post will focus on the Ayaka and Mina paths that were added on for this fandisc release.
The common route covers a summary of the events that happened in the first game and serves as a basis for just how much Ouro has 'remembered' about his own past with certain female personages and what he knows about certain figures vital to the story. It also covers the basic (very basic) resolution of Ouro's personal issues (in a really off-hand manner) in such a way that it means the writers are just 'letting you know' they won't be important to the two new paths. This ends once the summary passes the winter holidays and school is about to go back in session.
Shirogasaki Ayaka was the heroine who should have been instead of Akane. I say this because it is the same thing every Saga Planets fanboy said immediately after finishing the game... Akane had no business having a path in the game when Ayaka didn't. The tightly-knit nature of the group of heroines other than Akane and Ouro made the gap between Akane and the others as a heroine far too wide for a mere genkikko to really be considered a serious heroine in comparison. Ayaka, however, makes a strong impression from the beginning and involves herself (albeit in a negative way) with them from the beginning.
This path is hilarious. Anyone who played Rena's path will probably have some idea of just how hilarious a character Ayaka can be when she isn't putting her mind to it, but in her own path, I found myself giggling constantly. The way she and Ouro get together is so absurd, and the buildup to it is so... funny. Even after they became lovers, I couldn't take the serious stuff seriously because they made me break out laughing ever few minutes. I definitely rate this highly as an add-on path, and it is as long as the paths in the original game, so you can't say Ayaka was gypped.
Ayaka also has a 'what-if' append story that appears in the extra section after you finish her path. This what-if is just as hilarious in its own way, and it focuses on what she 'knows' pretty early on. This what-if dramatically alters her attitude toward Ouro in a way that is as hilarious as she was in her path, if in a drastically different fashion...
This is a path without any emotional catharsis, but in exchange you'll probably spend a great deal of time laughing.
As anyone could guess by the way she and Ouro interacted in the original game, Mina's path's theme is, in opposition to the ero-comedy of Ayaka's path, a straight out cute romance. As such, there isn't a whole lot I can go over here without spoiling details, sadly. I will say that the relationship formation is typical for this type of heroine, in particular because, unlike Sylvie, she is a lot less honest/familiar with her own emotions. For Mina, who has a strong sense of herself as a princess first and an individual second, dealing with emotions like those born out of young love is a bit awkward.
Unlike Ayaka's path, you probably won't find yourself laughing every few seconds, though Sylvie and the crew are as generally amusing as they were in the original game.
The append story for Mina is a straight-out after story, focusing on the aftermath of the two settling in as publicly (at the school) acknowledged lovers and dealing with Mina's homeland (which isn't as dramatic as you might think). As usual, the focus is more on the cute romance, so you won't see any real drama here (there isn't really a ton of drama in the main path either).
While the new paths aren't as straight-out powerful as the ones in the main game, I found myself more or less satisfied at Mina and Ayaka finally taking their rightful places as heroines. The laughs I got out of Ayaka's path made it worth shelling out the money to get hold of this, and I'm sure I'll enjoy the append stories for the other characters once I get around to replaying the original, lol.
Edit: Ok, so I couldn't resist. I broke down and played Ria's new path after all... I tried to stop myself, but my curiosity got the better of me... and I don't regret it.
If Mina is cute romance and Ayaka is ero-comedy, Ria's new path is a pure catharsis. For those who didn't like the ending of Ria's path (there will always be those who complain when the ending is bittersweet like that), this path is probably what you were looking for. I enjoyed it and came out of it feeling good, which is nice... but I do think that this path is pandering a bit too much to fan pressure. Well, since it is a fandisc, I suppose that was inevitable, in any case, lol.