Personally, I'm torn about the inclusion of Ezio Auditore.
As a huge fan of the Assassin's Creed series, it does excite me to see him playable on another game, particularly one that I liked. Now, I do say 'liked' for a reason, as I did like it. At one point in time. However, I'm afraid that the manner in which Soul Calibur has developed has honestly disappointed me, driving me away from the game. And yet, with the inclusion of the Master Assassin, I now find myself intrigued, and - scarily enough - attracted to the game.
On another note concerning the matter, I am glad that Team Soul decided to include a character closer fitting to the Soul Calibur franchise. Despite my love of the Star Wars franchise, I was a tad stumped as to why in Davy Jones' Locker they would add sci-fi characters (with bloody lightsabers!) to a game that looks, and feels, like it takes place in a more 'middle-ages'-type era. Really now, how can you witness a lightsaber being unable to cut through a steel blade and not see something wrong? But I digress. Ezio, being from the Renaissance era, definitely fits the slate better as a guest character.
So, am I glad about the inclusion of Ezio Auditore da Firenze? I am as of yet undecided. Will I still play the game? Yes, but only for Ezio. (And only as a rental.)