  We test the robustness and flexibility of the Chevallier-Polarski-Linder
(CPL) parameterization of the Dark Energy equation of state $w(z)=w_0+w_a
\frac{z}{1+z}$ in recovering a four-parameter step-like fiducial model. We
constrain the parameter space region of the underlying fiducial model where the
CPL parameterization offers a reliable reconstruction. It turns out that non
negligible biases leak into the results for recent ($z<2.5$) rapid transitions,
but that CPL yields a good reconstruction in all other cases. The presented
analysis is performed with supernova Ia data as forecasted for a space mission
like SNAP/JDEM, combined with future expectations for the CMB shift parameter
$R$ and the BAO parameter $A$.
