If you mean "test boosters", some work and some don't. Some work only for some people, and don't work for others at all. From what I've seen, Biotest's products usually do what they say they do, so buying some of their TRIBEX or ZMA might help you out.
As for your second question: substances taken that don't turn into drugs until they are inside you are called pro-drugs. "Not activated until they are in your bloodstream" is a false statement. They are not activated just because they are in your blood. They are activated by means of being metabolized. Most drugs are deactivated by metabolism, however pro-drugs turn into drugs after being metabolized. They are, in essence, just drug precursors.
They can be very dangerous and may have more sides than real AAS, sometimes not showing up for months. So no, they are not worth it, and no, they are absolutely not legal. An example of this would be androstenedione.