It disables the radio in between AP beacons, so unless there's a bug in the implementation it should have no noticeable impact to a quiet WiFi station other than saving a lot of power.

Seems safe, but it probably depends on the clock being accurate, so it can wake up on time for the next beacon, and the clock frequency is likely sensitive to temperature and therefore power usage.

If you're plugged into a wall wart, chances are the power savings aren't going to be too much; if it helps reliability (which should be easy to confirm), then it's likely worth paying a cent or two more a month. It's different if you're running from battery, though.