I agree with the cylcing comments. Just stop using it for awhile (a week or two), and you should start feeling more energized when you go back on.
By the way, I stopped using it for a month or so (just didn't feel it was necessary for a good workout). Yesterday I decided to take some before my chest workout, and I just couldn't get fatigued at all. I did a super intense chest workout, then did my tri's. Still felt super energized so I did my bi's and then for the fuck of it I also did my shoulders. I still felt fresh after all that, even though I was lifting more weight for more reps then I usually do! That shit is INSANE!
However, after shit wore off, I felt like a complete wreck. I'm sore as hell today.