Are fast passes in amusement parks worth it?

We frequent six flags and great america with the kids and have started seeing those fast lanes/passes quite a bit and the lines are getting longer.  I know Great America has those fast lane passes where you have a bracelet you pay for and are able to get through a fast lane for the bigger rides with long wait, Six Flags has the flash pass, and I see some other amusement parks have those too.  Anyone has used them before and are they useful for a family of two adults and elementary aged (but tall and brave) kids who ride roller coasters?  They are expensive though we can afford them if they will add significant enjoyment to our park visits.  We are considering getting those to accompany our season passes but not sure if it is actually useful and worth it.