I use them some of the time, largely depending on character concept and if a particular PrC fits that concept.
If it makes sense to move the character towards that PrC, then I do so. If not, I either stick with the base class they're in or multiclass using heroic classes as the concept requires.
Then again, it might depend on the base class. I can easily see a Fighter going into a PrC that focuses their combat skills in a particular area, or a Cleric/Wizard/Sorcerer going for a PrC to boost their casting prowess. Monk or Rogue or Barbarian, not so much.
