I've been running a maxed STH in arena for a while now. As long as he lives long enough to taunt (less of an issue, now) and you make sure to immediately focus down anybody who can dispel his taunt (QGJ, Asajj, the occasional Mace), he just plain wins games. The sheer amount of turn meter he generates for your team, if the rest of your team is half-competent, will just overpower any team you face. It used to be that he'd usually die in the process of winning you the game - most of my arena games went like this: 1st round, Han taunts, everyone else focus fires on enemy QGJ and kills him; 2nd round onwards, enemy team attacks STH, all my other guys get to go twice as often as enemy team, by the time Han died, it was my remaining four guys vs. the enemy's two least threatening guys, and squish.
The only issues I tended ran into were: 1.) teams with two dispellers (e.g. QGJ and Mace, or QGJ and Asajj) where my bad options were: wait to pop the taunt until they're both dead (by which point it's probably to late in the battle to matter much) or pop the taunt after only one is dead and *hope* the other one decides not to dispel or has used its dispel already; and 2.) extremely fast burst-damage teams that killed STH before he got to go (if they burst down anyone other than STH pre-taunt, STH's taunt was still generally powerful enough to win me the game with my remaining 3 toons).
Post-update, 2.) isn't really a problem anymore; I suspect 1.) will still be an issue but the meta changes may mean less frequent QGJ. So, basically STH was always awesome but he should be a little more consistent/less subject to bad luck now.
I will note that on Arena defense, he's never been amazing, because smart players can too easily make sure that they dispel his taunt (and it has a long cooldown) after which he's much less scary. I'd swap him out for RG, or use him together with RG, on defense.
One important note about STH: you want his Taunt to last as long as possible. This means, somewhat counter-intuitively, you *don't* want to upgrade his basic attack very much, because his basic attack generates turn meter for him, which causes him to burn through his taunt faster. Leave it at low levels (my STH's basic attack is level 4) and he won't take as many turns - I mean you're not really bringing him for his damage, anyways.
