Paper Mario Sticker Star was a disappointment. I really, really wanted to like it (I'm a huge fan of the rest of the series), but they strayed too far from the original action RPG formula and the new sticker attack principle, while interesting on paper (heh), is annoying in practice. If the stickers were used only for special attacks and items, it'd already be a huge improvement. Also, since they got rid of the experience points/leveling up feature in the game, it makes it a lot less interesting to fight regular enemies on the field. Hopefully, the next title in the series will be once more a flat-character cousin to the Mario and Luigi series.
Oh, and to go back to my backlog: I did finish Ys: Memories of Celceta and am on my way to finish Ys II on the PSP.