it is +2 con damage for every 6 power points, though it still makes it quite usefull, as the dc is based on your manifester level, and not the level of the power.

that aside, PWar is generaly consideret a pretty decent class, with quite a few interesting build options.
as for multiclassing, 20 levels Pwar is a viable build, and i would generaly advice against taking any levels that does not advance your manifester level as well.