Letrozole has a very bad reputation among most AAS users because sides are often too harsh.
Letro is commonly used PC to prevent/reduce gyno and during cycle to block test from aromatizing, and it is effective. Next to the loss of libido total sluggishness and joint pains are often reported, but these sides can be avoided. Here is why and how.

Letrozole is the most effective aromatase inhibitor out there, blocking almost all aromatization in the body. AAS users use letro when their own test production is very low, namely after a cycle. All the test that is in their body is exogenous. So letro inhibits effectively the aromatization of exogenous test. Letro blocks TISSUE aromatase enzymes and does NOT target the aromatization in the testicles.
Here is the point, after a test cycle our testicles are shut down and thus all the testosterone that is left is either in our bloodstream or tissue but not in the testicles. Since the testicles don´t produce testosterone, no estrogen can be derived from our own testosterone in the testicles. Therefore the body has absolutely NO estrogen because all the remaining aromatization is blocked by letro.

I have experimented a little and have gone up to 5mg of pharma letro (femara)/day for more than a month (extremely low estrogen IN THE BLOODSTREAM is confirmed by bw in my pic section. <6pg/mol = undetectable levels (in the bloodstream)).
The only thing I noticed was low libido after about three weeks in. I didn´t notice any difference when switching from 2,5cg to 5mg after 1 month (probably I wouldn´t have even noticed a difference when going from 1mg to the full 2,5mg because all the aromatase enzymes are already effectively blocked with 1mg of letro). There were multiple studies done on young, adult and old males and every study one can find says the same: the drug was well tolerated.
It is just that AAS users after a cycle don´t tolerate the drug well because their nuts don´t produce testosterone and thus no estrogen, and the remaining estrogen would come from aromatization of test in the body, which is blocked, leaving us with absolutely no estrogen.

If one could stack letrozole with HCG (to keep the endogenous test production kicking) the negative sides of letrozole would go away and the positive effects would remain, namely the blocking of the aromatization in the breast tissue and/or the prevention of additional estrogen derived from the exogenous test, because the testicles alone produce little but enough estrogen required to keep the harsher sides away. Letro does not target already circulating estrogen it just inhibits the production(aromatization) of estrogen. So don´t use a SERM when on letrozole, because then again, you will have NO estrogen and experience the harsh sides.