Letrozole seems to be very similar to Arimidex in the sense that it works by blocking the enzyme aromatase, subsequently blocking the production of estrogen.
It also has the same drawbacks as arimidex being (from stroidology.com ):
There are some concerns with using an aromatase inhibitor such as this during prolonged steroid treatment however. While it will effectively reduce estrogenic side effects, it will also block the beneficial properties of estrogen from becoming apparent (namely its effect on cholesterol values). Studies have clearly shown that when an aromatase inhibitor is used in conjunction with a steroid such as testosterone, suppression of HDL (good) cholesterol becomes much more pronounced. Apparently estrogen plays a role in minimizing the negative impact of steroid use. Since the estrogen receptor antagonist Nolvadex does not display an anti-estrogenic effect on cholesterol values, it is the preferred from of estrogen maintenance for those concerned with cardiovascular health.