I have had one done before, but be warned that depending on each person, you usually need 2 or 3 visits (close together) to complete the process and for it to be most affective, the first time round. I didn't realise that when I went and because I hadn't budgeted for more than one, it didn't really have any affect, as I couldn't keep up the treatments.
The experience itself was fine and not nearly as bad as you might think.
A good alternative, which I also tried and found really did work, is Gillian McKeiths 24hour Detox. It's about £20 from Boots or Holland & Barratt. You'll need to take it on a weekend when you're staying in though! It's a much safer and cheaper alternative.