If you are getting a DEFINATE percentage reduction , that has to be a good thing , though as Pavane says , check that the agent is Legit !
One thing to watch is that a LOT of Agents are NOT acting as agents per se , but are simply Sub Contracting your work ! We have had a number of occasions where (Usually due to the Stupidity of the agent) we have been aware that our fee , perfectly acceptable to us , was in fact only about HALF what the agent got paid !
On those occasions , a 20% commission would have been VERY acceptable .