Flash cm10 if you are not already on it, adjust your settings to match the ones in my screenshot above, and see if your battery usage is as good or better than mine. If it is, or even if close, then the extended cheap battery probably won't help you any.
Also to note the cover that comes with it, is super cheap, I had to request a replacement as my first one warped during shipping (early summer).
When I first got it, I noticed an improvement in battery life from the creepy battery usage I was getting (6-8 hours) up to 10 - 16 hours with moderate and sometimes heavy usage, on broken out v6 based Roms.
Any intensive application with the screen on (and forgetting to turn off 4g or GPS while using said app) kills the extended battery in 3-6 hours. Charge tinge is only 30%-60% longer than OEM.