They were some of the most unique role-playing games I've ever played. I don't know what it is about them.
The first one I played was YS Seven for the PSP. What bugged me was that when I saw the nice pictures in the artbook it came with, I expected to see beautiful backgrounds, and anime-esque cutscenes , and training minigames featuring the characters in a kawaii~ chibi form. I didn't see much/any of that at all. Now the game itself was good, but it wasn't what I expected, which is what bugs me the most about the games.
Eventually I've managed to... erm... obtain YS 1 and 2, and I found each one to be entertaining games. However, the games managed to be so easy. Even on hard, the enemies didn't really pose as a challenge.
Maybe YS 3 through 6 will pose as more difficult games.