Zass is a complete mind game character.
He does small chip damage all through the fight, until the opponent guesses wrong or does a mistake then comes most of his damage.
He requires patience, dedication and a great defense.
Interms of oki, personally I use 4B+K or 66BB mix that up with some lows. It's good enough.
I use several other characters, and to me Zass is still the one that requires the most skill from me to use.
That's why I appreciate the guys that use him, I have said it before I will say it again Zass has the options. But he requires a tremendous amount of skill from the player to win with him...Plain and simple.
Yeah, this is my opinion on Zas as well. I actually used him (along with Setsuka) as my main on SCIII, and I initially ignored him in SCIV because they changed him so much, and I wasn't really getting into him enough to adjust to his completely new playstyle, so I mained Setsuka instead and decided to switch my Main Character and Secondary character around.
Unfortunately, over the last several months my joints have been getting progressively worse (they've been bad for several years, but the last year has gotten a lot more severe). Basically, I've got a full-body joint problem which so far I don't have a decent diagnosis for (multiple doctors have tried to find out why all the joints in my body pop and crack anytime I move them).
But anyway, enough about that. My point was that, due to the immense pain I'm in, I can't go all-out with the controller as much or as often as I used to be able to, which is a severe problem with a character who has as ridiculous a command list as Setsuka, since most of her controls are far more straining then a lot of the other characters. Due to this, I decided to pick up Zasalamel again some months back (I've used a bit of everyone but I was mostly using Setsuka), and ever since then I can honestly say that I'm far more dangerous with Zas then Setsuka. I actually prefer this Zasalamel over SCIII's version now.
Zasalamel is a perfect example of a technical character, in my mind, since you actually need to have a lot of skill, patience, and defence to use him, just like you said in your comment above. I can honestly say I haven't seen many people using him well, even though I'm sure there's good Zas players, and a lot of this is likely due to the fact that Zas can't rely on abusing various mix-ups all the time and being overly aggressive. In order to use Zas effectively, you really have to make the opponent mess up, pulling them into your traps and then taking advantage of the ability Zas has to control the distance between himself and his opponent. Though I do still use Setsuka anytime my painkillers are giving me a little relief, I also find Zasalamel to be a much less painful character to play, due to the defensive/calm nature of his style, and also due to the fact that his move list is nowhere near as unneededly complicated as Setsuka's.
It's true that Zasalamel doesn't have a lot of low attacks, and at first glance he appears weaker, slower, and less safe then many other characters, but in the hands of a player who knows what they're doing he can use his tactical control and distance management abilities to use the enemy's own strength and aggression against them. Against a lot of people, all I need to do is wait for them to try and attack rush me and then I can trap them in a chain of various pull and push moves. The trick with Zasalamel once you've got a hold of your opponent is knowing which attacks have good priority after you've pulled or pushed the enemy into specific positions. Together with his ability to control the distance between himself and his opponent, as well as pull them off guard, Zas also has very effective throws, in my opinion.
EDIT: Oh, and I just thought of something else I wanted to add now. It's good to see that other people have discovered how to develop a good gameplan with Zasalamel as well as myself, since I actually had to work all of this out myself without any guidelines (though I tend to always play like that), so I wasn't sure how many other people had found a decent way to use him. I'd watched several tournament matches from around the world on youtube, and all of the Zasalamel players were really not very good.
I also happen to find f/f/A+B (66A+B) to be an effective move, btw, especially as a punisher against an agressive opponent when I need to quickly close the distance and cause damage to an opponent whose making a mistake. The move is also a good choice if the opponent is blocking low a lot, something which I know how to force/trick my opponent into doing.