Yes, I have. And it made an amazing Feta like cheese. At least to me it smelled like Feta. I had some Organic Store bought Kefir, without the grains obviously. So I took some Organic Store bought milk poured it into 3 empty 32 ounce yogurt containers and added about 5 - 10% Kefir to them. I left it on the kitchen counter for 3 days. The first two days it continued to look and smell like milk. However, on the third day, a beautiful curd was formed, I mean it was perfect. No need for further cooking or anything. So I pressed it and brined it and am enjoying it as Feta right now. It has that well developed Feta taste that is difficult to describe in words. Gamey perhaps?
In any even I really love it, and will certainly make it again.