I have to say I own a lot of Mac brushes and now have quite a few Sigma brushes and I love them just the same! SIGMA BRUSHES :: Dupe List for Mac Brushes. I have a bunch of both, and it is true, they ARE great dupes.
I def prefer MAC shadows vs my UD palettes. The MAC colors that dupe for Urban Decay's Naked Palette (you can search for tutorials on different makeup looks for the Naked Palette and use either Mac or Wet N' Wild Dupes). or get the actual palette.