We did an evaluation a few years ago, comparing Belt to Chain for a drive system. There are good and bad with both options. The report is linked below.
In 2013 we used 15mm belts on an 8 wheel drive with no problem. We had two belts from the gearbox to the center-most wheels, and another belt from that wheel to the outer wheels (total of 4 belts per side). These were Gates.
For 2014, we are direct driving the center wheel on a 6 WD and have one 9 mm belt going to front and one to the back (total of 2 per side). We have seen some wear on our prototype robot and it has probably 100 or more hours. So far no wear on the competition bot but it has limited practice time and about 45-50 matches on it. We have VEXPRO belts on the prototype robot and Gates on the Competition robot.
http://www.chiefdelphi.com/media/papers/2216