BIG FAT WARNING: If you're going to relay your Raspberry Pi / Arduino to mains electricity, be very, very careful! Mains electricity can KILL. Make sure you learn about safe shielding, connecting and grounding practices. Above all, be extremely cautious! I can't stress this enough. EDIT: For more information: http://tubelab.com/Safety.htm
Better yet: Don't connect your Raspberry Pi / Arduino at all to mains electricity unless you use an opto-isolator, or you have training in designing such circuits. The usb relay is NOT designed for this! See the picture: http://www.circuitgizmos.com/products/cgu451/cgu451.shtml In a device intended for hybrid hi/low voltage there is a clear gap between the high and low voltage components, and an isolation transformer…
An isolation transformer would be of absolutely no use here and the optoisolator would provide the same functionality that the existing relays on that board do.
I can't see how the PC board is laid out so I can't verify if there is inadequate separation between the control side and the isolated side of the relay (even so, you only need about 5mm separation to be minimally safe at 120V), but barring that, it's fine!
Yes, this particular device is not explicitly designed to deal with all the hazards of switching a high voltage load, but I don't see anything that leads me to believe that it's unsafe for that purpose.