You see, I read more stuff about REPLACEMENT belts breaking than anything else. Not only could it be inferior, but more to the point, just because they SAY it is the correct size does not make it so.
KYMCO and SYM use proprietary belts made for them by BANDO, and so far I have heard of them often lasting 12K- 15K miles (which I personally do not want to chance. Mine get changed out at 5500-6K miles). The OEM belts do not generally seem to give problems, unless left on way too long. The type of riding you do also affects belt life - two up seems to be the hardest on belts, but my stop and go NYC commute can not be good either.
Over on the Chinese scoot forums you read about belts breaking in the first 300 miles all the time. So stay away from Chinese junk belts.