The knock-offs generally work, for the most part. They do usually take a charge and power your device. But, if you want it to also work reliably and to last for many months, buy Joyetech or Janty.
I'm still using the first Joyetech battery I ever bought, after 2+ years. Of course, I have lots of
batteries. So, I'm sure it would be dead by now if I only had a couple and had to use it every day. Batteries can only last so long.
That being said, most of the knock-offs really are adequate. And, it's great to have lots of colors, sizes, and patterns. With knock-offs, you can afford to do that.