I went to a Harley Street clinic for a £90 colonic irrigation treatment back in 2003. It had practically no effect on me.
I had read a Guardian weekend magazine article about people going to Thailand and getting the serious colonics done (where they do something like 1-2 colonics per day for a week, using coffee and such to really dislodge what is stuck up there). I read stories about people who had dislodged marbles, elastic (rubber) bands, wads of chewing gum they had swallowed decades ago, undigested meat, etc. etc. The article even showed graphic photos and I was really up for purifying my colon thusly.
But, my experience was not life changing in that respect, and I'm not sure why, but I wanted to reply in case anyone had the high hopes for it that I had, that were not met.
Having said that, if I had the chance to do the heavy duty Thailand-style series of colonics, I would give it a try, in hopes of better results.