How well do slave flashes really work?

I have a Cannon A630 and am considering buying a slave flash for some amature hobby photography. How well do these types of flashes really work and are they worth the investment?

ALL SLAVES ARE GOOD, SOUND ONES, LIGHT ONES OR RADIO ONES; THE MAIN THING IS WHERE YOU PLACE THE OTHER LIGHT TO SYNCHRONIZE WITH THE MAIN ONE.

Wein makes several slaves that I have used over the years, depends on what you need it for. I have never had a problem with theirs and the prices are very good.

Slave flashes could use as background lighting,so that you won't seen shadow when taking portraitures.