To clarify a bit, if you're accustomed to how easy it is to use US$ in countries like Panama and Ecuador, which are fully dollarized, or Peru which is a partially dollarized economy, Chile is a different animal. Basically, you cannot use US$ for cash transactions anywhere except (as noted in previous posts) possibly to pay your hotel bill (and only then if they're billing you with a factura de exportación which exempts you from paying the VAT).
That said, you can readily change money at the airport and in many places in Santiago. Most money changers do not charge commissions and the exchange rates are competitive, with very narrow buy/sell spreads. As previously noted, the US$ is weak against the Chilean peso, but this has nothing to do with the money changers and everything to do with the high price of copper and the US government's idiotic fiscal situation.
As a carrier of a Canadian bank card I never seem to escape outrageous ATM charges in Santiago. If this is a concern, you should bring plenty of cash and change US dollars into pesos as required during your stay. You will likely be spending most of your time in the upscale neighborhoods of Las Condes and Providencia, and there are plenty of money changers to be found in these areas. For instance, try Av. Pedro de Valdivia just north of Av. Providencia.