  The ergoregion instability is known to affect very compact objects that
rotate very rapidly and do not possess a horizon. We present here a detailed
analysis on the relevance of the ergoregion instability for the viability of
gravastars. Expanding on some recent results, we show that not all rotating
gravastars are unstable. Rather, stable models can be constructed also with
J/M^2 ~ 1, where J and M are the angular momentum and mass of the gravastar,
respectively. The genesis of gravastars is still highly speculative and
fundamentally unclear if not dubious. Yet, their existence cannot be ruled out
by invoking the ergoregion instability. For the same reason, not all
ultra-compact astrophysical objects rotating with J/M^2 <~ 1 are to be
considered necessarily black holes.
