In my less knowledable days I tried, nitrotech, celltech, gakic, hydroxycut and meso-tech. Here are my reasons- High price, BS advertising, shady "university research, there are WAY better supps that cost less and are proven effective, plus pretty much all their protein taste like crap when compared to other companies.....and then there is the GNC connection. They have also been accused of "pixie dusting" ingredients in their products...hence all their products are "Proprietary blends"