The amount of time you rest between sets can significantly affect your results. Rest up to three minutes between sets if pure strength development is your priority, and one to two minutes if muscular endurance and tone is your priority. Three minutes permits the muscles to recover from fatigue so that you can generate enough energy to perform another maximal lift on the next set. Benefits are not discreet. That is, there is carryover from one style to another, so that if you rest just one minute between sets, you will still increase endurance and tone, and if you rest three minutes between sets, you will still gain endurance and tone. I recommend one to two minutes for most people. More than that and you may end up spending more time chatting with others in the gym than getting down to what you're lifting for in the first place, that is, getting stronger. The guiding principal with weight lifting is volume. That is, the equation sets x reps x weight (in pounds). You can see that the more work you do the more benefit there is. If you take a lot more time resting than lifting, then you minimize your benefits. Take breaks between sets for sure, but get back to work as soon as the muscles are rested.
It should be noted that in theory if one was to consistently suppress your natural estrogen levels for a long period of time, this would negatively impact your health, including your cholesterol. Due to the ability of Letrozole- to inhibit estrogen so much, this should definitely be a concern to most users. However the research that has focused on the relationship between use of letrozole and cholesterol levels is rather inconsistent in it's findings. Many studies have concluded that the compound is detrimental to both a user's HDL and LDL cholesterol levels, while other research has found no link. Obviously individuals are best served to monitor their cholesterol while using any compound via blood tests however barring that, letrozole should simply not be run for extended periods of time if at all possible. Doing so could cause serious medical complications.
Along with the issues related to blood lipids is the fact that many users complain that their libido is dramatically reduced when using the compound. This is related to the fact that estrogen is partly responsible for the regulation of an individual's sex drive. Since Letrozole- is so potent it can often drive estrogen levels too low and this inhibits a user's libido. To avoid this users can lower dosages, but some anecdotally report that even extremely low doses of the drug can cause problems. If this is the case a less potent compound such as exemestane or anastrozole may be a more appropriate option.