I've been using Nitraflex for ca. 6 months and I found it a really nice pre-workout. Nice pump and focus without any side effects. I mix 10 g of powder with some creatine, glutamine, and 1.5 g of kitchen salt. This particular flavor is not my favorite though; I prefer green apple, orange, or pina colada.
This is one out of few preworkouts I can take and still fall asleep at night. It is an ok pre workout if you are an average lifter. However if you are an advanced lifter and you like strong pre workout formulas, this product is not made for you. I don't know about the testosterone boosting it claims, but it just give me a little bit more energy and endurance than usual... but not enough to me for a solid outsanding pump, just a reasonable average energy boost but less focus and not as long lasting as other pre workout like Mr Hyde or Pre Kaged.
I used a similar products from the same company before, which I really liked but is no longer available on iHerb. So I tried this, and it's a bit disappointing. I only use 1/3 the dose and mix with other things...the previous incarnation was fine but this is barely passable.