Now Ive made a first try with the Zest-it medium and here are some of my thoughts.
The citric smell. Many people seem to be bothered by the citric smell and even experience skin irritation. I am not allergic to citrus fruits and since I love how it smells this product hits the spot for me!
Zest-it is more liquid than Liquin and it reminds me more of the regular mediums on the market. I started to paint with standard mediums and then switched to Liquin so I am used to work with both kinds. When using a medium on a bigger area I would prefer the more liquid ones. I avoid mixing different mediums in the same painting though.
Drying time. Liquin is extremely fast drying compared to Zest-it. With Liquin it can take as little as four hours and the painting has dried (depends on how much you use of course), but when I tried Zest-it it didn't dry overnight. I tried to go over it with a new layer way too soon, not recommended! If you have little patience or are in a hurry Liquin is unbeatable.
Toxicity. You can read that Zest-it is non-toxic on some pages, and that it is dangerous on others. Google it and make up your own mind about the product. I think it is less dangerous than Liquin at least, and it feels safer to use it in a small studio like mine.