Peercoin has been successfully attacked. The reason these PoS coins are still up and running today is because they either use checkpoints (which makes them 100% centralized and pointless, which is the case of Peercoin), or they are continually making changes which makes attacking them annoying or hard, but is just another form of centralization (they depend on the continuous updates that everyone must accept blindly, as opposed to a stable protocol that rarely changes. Someone compared this to the whack-a-mole game).