Hi,
I did the Jason Vale diet about a year ago. I did it for two reasons - detox and to lose a few pounds before a friend's wedding. It's pretty difficult at times, but it definitely worked well for me. Some of the juices, especially the ones suggested for the first few days, are pretty awful. I made a few amendments to mine to make them a little more tolerable.
By day three I was getting quite bad headaches and generally felt a bit "eugh". It passed fairly quickly though.
Some if the juices are quite high in sugar as they contain so much fruit. Be a little careful here and keep an eye on your blood sugars.
Good luck!
Em