I like to think of gold onlays as conservative crowns. That's because i don't have to remove nearly as much natural tooth structure to do them. And no restorative material is as good as natural tooth. An onlay, as opposed to an inlay, covers at least one cusp. You can think of a cusp as each little "mountain" or bump on your tooth. Some people say that gold is not aesthetic (despite loving Jack Sparrow and his golden front tooth), but I just love the way the yellow compliments the white surrounding the glassy smooth margin. Then again, often you can have the best of both worlds where you can see the beautiful gold if you're looking on purpose, but it doesn't show with a "camera smile."