The Saito path's main theme is "falling apart;" that's how it comes off to me from everything that happens, at least. If not that, then "change that one cannot control." I admit, Saito's character archetype is one of my favorites; I think he's really quite cute. My main issue with it is that, compared with the other paths' main characters, the heroine really comes off as bland. She literally has no thoughts of her own compared to the better heroines in Okita and Harada's paths. I think this is because Saito would prefer a quieter, more subdued, woman and the game made her so.
Some thoughts on the other paths - minimal spoilers follow, but nothing game-breaking.
I'm saving Hijikata's for last because it's by far the longest and also seems like it was meant as, dare I say, canon.
Chikage's is more interesting than anything. The "Normal" ending here is happier than the "Happy" ending. The "Happy" ending was probably the saddest moment in the game for me.
Okita's is the tragedy, but also has some very interesting parts to it that aren't elaborated on in other paths. This is the second longest path.
Harada's path is unique. That's all I'll say. He's the only character that doesn't do something and the path is quite different from the others because of his motives and relationships. He is very, very sweet. Not passionate like Heisuke, simply kind and devoted. This path also is the only one that gives development to the final member of Chikage's group.
I'm planning on doing Heisuke next. He's a sweetie, but his voice is horrible. I think I'm going to mute it.
I have to say, all of the antagonists are actually quite well developed, with reasons that aren't obnoxious or pathetic. I was quite impressed with how well they were handled, and how their development is spread across the various paths.