Hi John,
There is some evidence to suggest that supplements may have a small effect in reducing blood sugar levels, but the reductions are minimal by comparison to what can be achieved by prescribed medicines. As well as cinnamon, you might like to have a look at gymnema sylvestre which has been used as an anti-diabetes treatment in Indian ayurvedic medicine for thousands of years.
There was a study in 2005 into alpha lipoic adic by the University of Arizona Medical Dept which proved that ALA can be beneficial in reducing insulin resistance, but only when used in conjunction with endurance exercise training. I have strong doubts about the effectiveness of evening primrose as a diabetes aid. The Arizona study showed that ALA is a powerful antioxidant which can remove the barriers that prevent insulin from transporting sugar into the body's cells. One of the main barriers is phenylalanine, which is present in concentrated form in evening primrose!! Hmmmm!!