To Cleanse or not to cleanse..that is the question!!

Has anyone done a cleanse before?? I keep hearing about them and how great they are for you. I have big plans on trying one after I run a marathon in October. I have heard good things and not so good things about them. Could be the product, could be the person...I don't know!!
If anyone has any experiences they don't mind sharing I would love to hear about them!