I particularly like the way that painted bowls, once worn a bit, show up the tool marks in the wood. This is also really nice with carved bowls.
My latest bowls presented me with a bit of a dilemma. They're turned from Sycamore that was very green and the wood is pretty bland, so I was very tempted to paint them, but my wife has been telling me to leave some natural for a while now. Naturally my beloved (I have to call her that or she hits me) won the argument and so here they are in their birthday suits.