Antonio was so helpful, adjusting our bikes and helmet straps, answering our questions, and even providing a local map and marking the best places to ride. We happened to bike on a Sunday when one of the main roads along the river was closed specifically for bike riding -- I'm not sure if this is a weekly or monthly event. But it was great! Even without the road closure, renting bikes is a fantastic way to poke around Santiago.
ONE TIP THOUGH: The shop is exactly where it's promised on the map (a few blocks north and east of its location a few years ago next to the river) but we couldn't see the name of the business from the street. The building looked closed up; we only found it because we made sure to RING THE BICYCLE BELL NEXT TO THE DOOR. Antonia said he's always there, they just keep the doors closed to cut down on theft, I think.