Stormboyz are pricey Ork boyz who lose out on ObSec
and an attack for having native Deep Strike and being just fast as hell. There's a lot of value in the native deep strike in 9th, because you can bring small units onto the table to achieve objectives, but kommandos are cheaper for this role.
So it's really about being a faster Ork boy. They don't get the attack for being 20+, but this is a small blessing; you don't have to feel compelled to run large mobs. I mean, I would gladly take the extra attack, but you take what you get. So using that speed is the key. Either you get a quicker charge (including from Deep Strike), or you're able to protect the mob by hading it more and utilizing the speed to jump out onto something.
The cost is a big detriment, as is the lack of ObSec
in 9th. They are no more durable than a boy, so each casualty is more expensive. Its because of this that we have to be more canny with Stormboyz, using their speed to protect them and put them in positions where they can excel, and not just use their bodies to bully the board like we do with Boyz.
As for Zagstruk, I used him pre-codex to keep three 10-ork mobs of Stormboyz on the table. I think this is exactly how to use him now; smaller Stormboyz are easier to hide, and he can aid units with his meaty legs. His downside is that he takes an HQ slot, which have become very valuable to Orks in 9th. I don't think he is worth the slot, unless you've somehow got an empty one.