I've never seen anyone tell someone they must use any Biotest product.
I've worked in the supplement industry long enough to have learned to not trust most brands, though. Biotest is one of a few brands that I do trust because of what they do behind the scenes that never even makes it on to these forums. I have not seen one instance where they have sacrificed quality in order to increase sales.
I've worked with raw material suppiers and know that top quality protiens cost companies more than the discount proteins consumers purchase online. Even with volume discounts and all that large volume manufactures receive, it's not economically possible to buy a 5 or 10 pound package of high-quality protein for the prices that are commonly found online. So, I know these are not top quality products.
This doesn't mean that they should be taken off the market or are dangerous in any way. They just are not something I use or recommend.