I have had this running outdoors during daylight hours for over a month and have had no problems whatsoever.  The sound is relaxing and the water level is easy to maintain even in windy conditions--I top it off every couple of days.  Since there is no on/off switch, I recommend buying a timer so it doesn't run 24/7 and (more importantly) you don't accidentally leave it off for too long, allowing mosquitoes to breed in the vessel.  This has proven to be one of the most satisfying purchases of the summer.  Highly recommended!