I have been experimenting with urethane belts myself.  At first
blush these seem to be a great alternative to leather and rubber.
For one, the urethane belts don't slip.  They also are great if
you have a three-step pulley to change speed on a 10-tune roll.

My biggest issue with them is that they vibrate while running.  On
one machine I restored it vibrated itself off the pulley.  On a
Coinola "C" I restored the customer called and had the urethane
belt replaced with leather.  We could actually hear the belt vibrate.
I included a YouTube link that will show what I am talking about:
http://www.youtube.com/watch?v=3DXHKYnfungqM&feature=3Dplcp 

Tom Neuhaus - The Mechanical Music Workshop
Milwaukee, Wisconsin
info@themechanicalmusicworkshop.com