I have always had good luck with the factory polaris 1066 belt on my 850 ultra. Yeah, they do blow up from time to time, but i have had better luck with them than any others.
If you are having trouble with belts, make sure you check clutch alignment, runout, is the sled shifting our and holding the RPM you want? gearing can also play a role here. Take a sharpie marker and draw a line from the center of the primary to the outer edge, then go and ride like normal. Check and see how far the mark has been erased. If its not erased most of the way up the clutch sheave, assuming its holding the RPM desired, then your gearing is too high. This causes excessive belt slipage at lower RPM, excess heat, and wear on the whole CVT system.
