Well my honest thoughts on them, as a layperson / observer is, many aren't worth a dime and you get fast weightloss that eventually comes back in part or full.
Colonic irrigation might be one approach, rather than taking some pretty nonsensical thing like isagenix, where you consume strange liquids and pills for a week.
However you do get the other diet based types, not shake or shake and pill based types, where you get told to eat 1 or 2 foods for 2-3 days, then some other foods for 2-3 days, then one day of pretty much one thing, with a proper breakfast and essentially an eating plan, cobbled together daily choices, that for the potential consumer, must make no sense at all.
Somwhere on the net is a diet plan for workers at Tata Motors in India, which I think had 2 days of little protein content at all, then one day of just lots of Chicken and Tomatos I think, then a day mostly full of Bananas and Potatos, or something strange like that.
Just made no sense to me at all.
Things that might help you, would be increasing things like Wholegrain fibre consumption, or things like Psylium or Senna. You could drink more Water, or eat Figs and Prunes, or have things like Fibre Sure, Benefibre, Metamucil etc, as such things can positively affect bowel function, or have a laxative like effect if you ever feel a little uncomfortable or "bunged up" for want of a better phrase.
I certainly would not reccomend you take any cleanse involving pill based consumption, as they are probably very pricey, certainly I think Isagenix is from what I read some time back, and if you really are keen on it, do the reasearch on ones that are purely liquid based and make sure you understand exactly what the ingredients are, if you're not sure on all of them, make sure you know what constitutes good levels of ingredients and also make sure the price is comparable to what you're getting content wise.
Though those are some of the hallmarks I often espouse in relation to all supplements, especially if they contain herbal or chemical content.
Hopefully that goes some way to helping you make a choice either way. I'm sure others might have something to say, but good luck