Good old PKS of course still exists, but as I know not on longhaul routes?
PKS doesn't exist in terms of being national - but instead, it's a patchwork of independent companies all operating under the PKS fare rules. But yes, they can do long distance - there's a bus from Poznan-Przemysl for example. Connections tend to be very odd though - but that's because of the lack of a national operator.
rozklad.com.pl is your friend :)