Me I have it very regularly.
I'm a Beauty Therapist & a girl at work does it.
It's fab!
I have trouble "going" sometimes & find it helps to move things along nicely.
It doesn't hurt but can be a bit uncomfortable if you are constipated. It feels like the sensation you get when you really need to go to the toilet. As you're not sat on a toilet you may find yourself tensing up to hold on to it, but you need to try & be as relaxed as poss to help it along. They use warm water & cool water. If the stools are hard & you are a backed up, then it can be difficult to soften so you may have to go back for another a couple of weeks later.
You'll feel great afterwards, but its a myth that you lose alot of weight afterwards. If you were constipated then you might lose a lb or so, but not much more.
It is also normal not to go for a few days afterwards as you've been "cleaned out" effectively.
Sorry if you've found this a bit gross, but it's the only way I can describe it.
I'm all for Colonics they are the way forward.