At first I wan't too hot on this ability, but I waited until I got my counter buffs up to +11 and started using it.
It wasn't until after getting aggro by a tough crawler after I had been soloing a tough Exoray that I really appreciated it. With less than half my hp left, counterstance allowed me to nearly kill it. It still killed me, yes, but it only had a sliver of hp left in the end.
And bear in mind that countering is uneffected by paralysis and blinding. Counterstance can keep you in the game even if you can't hit on your normal turn.
It's not good against everything since it can't counter ranged attacks, but against most mobs, its a life-saver.