yes it is deadly and for months (even before the buff from patch) i phantasize about using it.
imagine a protoss who is at 2 bases, got 2 collossi out and thermal lance upgrade researched:
your 5 sentries which are at full energy make some void rays as hallucination and the big battle starts. making collosi only,
your opponent would know that it is impossible to have 6 colossi by 9mins or so^^
i also think that with easier acess to detectors it might lose its powerfullness fast, i'm not too sure about it