I have played with my Stormwall quite a few times now. I have won every game that is was destroyed. I lost all but one game where it lived. But that one game I won and it lived was my most recent. So I think I broke the streak.
As to the collosal itself I like to stick it right up in the middle and force the opponent to deal with it or try to go around. Neither of which is easy. It definately adds a new dimension to the meta, but I don't see it as game breaking. It is alot of power in one package and casters/support/etc that are good at buffing single models do well with the Stormwall.
OTOH I have played against the circle gargantuan once (his first time playing it) and rolled it up pretty easily with a unit of Bane Thralls and Tartarus.
My friend recently got Hyperion and I have played against him twice. Both times I was pretty much able to avoid him and get the objectives or caster kill. But since he just got him he hasn't played him much, either. I expect both models to be much nastier once the people playing them get alittle experience with them and build a list to use and support them better.