I am no expert on this, as I get colonics only occasionally. However I always do feel a little cleaner and lighter afterwards. I can also watch and see what gets washed out, and there is definitely stuff coming out. I mainly do colonics after a liver flush.
I have never been asked to pay in advance, would rather find myself another place. But don't know what the common practice is in Houston.
I don't think colonics are the end all and be all of colon health, just a peace of the puzzle. It cleanses the last five feet of the intestine (colon), a very important part, but our intestines are many many feet long. Therefore a lot of cleansing has to happen from the top down. I prefer to spend my money on bowel cleanses, juice fasts etc.