A good healthy dousing of Comet Kitchen Cleanser, coupled with a new S.O.S. Pad.

Totally Kidding

Since the Matte Gun Metal finish seems to scratch rather easily, I've strayed away from any of the harsh, common wheel cleaning products found at any of the local automotive parts/supply stores.
I have found though that a product sold at Walmart made by "Purple Power" called "Citrus Cleaner" has worked for me.
Be sure to use a MF cloth though and always use either a separate bucket for the wheels and wheel wells, then you use for the painted surfaces.