Excellent air-conditioned buses connect Rantepao with Makassar. Elsewhere you're looking at pretty clapped-out local buses that stop every few minutes. There are some decent long-distance bemo (minibus) services, particularly on the road across Central Sulawesi connecting Luwuk and Palu.
Shared Kijang (a type of taxi) are also common; these are quicker than buses, and cost a bit more, but aren't necessarily more comfortable.
In towns minibuses called mikrolet or pete-pete are the main mode of transport for locals.