I've done it before. I don't know if I am detoxifying or "cleansing" (I hate that word, it's so cutesy! Either it's cleaning or it's washing, but cleansing is too dainty for me.) but it was an interesting experiment.
I have fasted 3 days on just water, done the Bevery Hills Diet in the 80s for 6 weeks, which has 10 days of just fruit I think, and done juice fasting and all day watermelon fasting.
It feels kinda interesting. It is something of an altered state, especially after day one of the water only. Your brain needs glucose the most, so it gets a bit odd when you don't eat any.
I do it once in awhile for a day or so just to jolt my system, but I don't want to harm my fitness level, so I don't do the longer ones anymore. I believe it can't hurt and if it helps, then wonderful.