It's difficult to know the frequency of planets in stellar systems, because most of our current methods of exoplanetary detection work much better for very massive plannets revolving very closely around very small stars. Systems like earth/sun are very difficult to detect. This intoduces the old astronomer's nemesis, the "selection effect", where observational considerations tend to exaggerate some kinds of results while biasing against others.

In addition, as posters below point out, there are probably many possible episodes in a star's history that may reduce or eliminate planets. However, I do believe the general consensus among astronomers is that planets are an almost inevitable result of the process of star formation, and are probably quite common, especially among the very numerous, ordinary, well-behaved stars like our sun.

Still, the fact is, we really don't know for sure.