Biotest products are interesting because they really appear to put a lot of research into them and (seemingly) high production values. They also appear to follow research paths not heavily tread by others.
I also like the fact that their standard answer is that they understand their products don't work for everyone, and if they don't work for you, stop taking them. I like the attitude.
Biotest SPIKE works amazingly for me. It gives me precisely what they say it does (lightning mental acuity, energy without feeling like I'm on a stimulant, and confidence). I like SPIKE and take it before I engage in high-stakes negotiations.
Biotest M didn't seem to do anything for me.
As to the product at hand, Biotest Carbolin19, I have to say that it did appear to work for me. There wasn't anything dramatic (like superdrol or M1T) but it did appear to do precisely what it was supposed to do (act as a bridging product, preserving gains and aiding in body composition). Like most people taking supplements, there is no double-blind test. I took carbolin19 while doing other things and my gains could easily be attributable to those other things....but my gut tells me that it worked for me. I'd be taking some now but it's so freakin expensive.
I think Biotest gets a bad wrap from people who expect instant dramatic gains from their products. They don't claim their products will make your muscles EXPLODE after a week's use and it seems unfair to hold them to such a standard. They create interesting stuff (their GROW! protein bars taste wonderful).
My experience with Biotest has been:
GROW! Protein Bar: A+
SPIKE: A+
Anti-E M: C-
Carbolin19: B