  We study generic single-field dark energy models, by a parametrization of the
most general theory of their perturbations around a given background, including
higher derivative terms. In appropriate limits this approach reproduces
standard quintessence, k-essence and ghost condensation. We find no general
pathology associated to an equation of state w_Q < -1 or in crossing the
phantom divide w_Q = -1. Stability requires that the w_Q < -1 side of dark
energy behaves, on cosmological scales, as a k-essence fluid with a virtually
zero speed of sound. This implies that one should set the speed of sound to
zero when comparing with data models with w_Q < -1 or crossing the phantom
divide. We summarize the theoretical and stability constraints on the
quintessential plane (1+w_Q) vs. speed of sound squared.
