We currently have two with us.  We are with KVC.  It is our 5 and 6th placements.  It is interesting and David sums it up much better that I can.  

"The biggest challenge for us has been the “system”. We have no input, no control, and are totally at the mercy of the judge. The decisions they make are bizarre to say the least."

I know of a family member of mine that had a newborn till 18 months and the mother eventually got custody back.  Said family member seems to have forgot it was a possibility from the start that the kid was on a reintegration plan from day 1.  I know its hard, but they spent tons of time advocating against the mother and just not accepting that it was out of their control.  They say theyll never do foster again...  
So it can be quite a burden if you let it.