I've just started learning. I jumped in with both feet and fast learning from gavin webber's youtube video recipes and reading like crazy. In the time, this past 6 months I've made 50, approximately 4 gallon batches only using store bought pasteurized and homogenized milk. I haven't thrown away any cheeses that I made so far, though I have favorites and average. I had one batch that foamed and curds floated badly, looking very out of character, and most likely the milk was contaminated, so I tossed that. If you have the equipment, the press, mold, the cheese cave for aging, things don't go bad overnight, if you pay attention, wash the cheese, or wax it, or vac pac it, sterilize everything before beginning each time, you get cheese. Some is just better than others. So a learning curve, I learned as I went. In March I made 12 hard cheeses, April 15, May and June another 11, add soft cheeses, curds, motz, cottage, ricotta. There is a cheese making thread in the dairy area..somewhere, you can read about it. It's awful fun, really.
I took on the headset of doing what dairys and shepherd families have done for thousands of years, using milk in a way to preserve it, to feed their families.