I did three over the course of three weeks, and while the person administering them said that she could see candida coming out of me (she said she could tell because it looks like "snowflakes"), I didn't notice much of a big improvement in my overall wellbeing. My naturopath doc swears by them though.
Then again, I had already started doing enemas by the time I did the colonics, so maybe that's why the effect wasn't so dramatic. Enemas on the other hand are a must IMO.