  The dynamics of expansion and large scale structure formation of the Universe
are analyzed for models with dark energy in the form of a phantom scalar field
which initially mimics a $\Lambda$-term and evolves slowly to the Big Rip
singularity. The discussed model of dark energy has three parameters -- the
density and the equation of state parameter at the current epoch, $\Omega_{de}$
and $w_0$, and the asymptotic value of the equation of state parameter at
$a\rightarrow\infty$, $c_a^2$. Their best-fit values are determined jointly
with all other cosmological parameters by the MCMC method using observational
data on CMB anisotropies and polarization, SNe Ia luminosity distances, BAO
measurements and more. Similar computations are carried out for $\Lambda$CDM
and a quintessence scalar field model of dark energy. It is shown that the
current data slightly prefer the phantom model, but the differences in the
maximum likelihoods are not statistically significant. It is also shown that
the phantom dark energy with monotonically increasing density in future will
cause the decay of large scale linear matter density perturbations due to the
gravitational domination of dark energy perturbations long before the Big Rip
singularity.
