I was looking at some knock off Dewalts last night on Amazon. The fakes are half the price of the genuine article.
I think they would be great if you bought a combo kit that had some low amp hour batteries and were looking to expand. If they malfunctioned you would still have the stockers to fall back on. I'm not sure I would want to rely on them exclusively, especially for professional use which I would be using them for.
I have seen one knock off battery out of probably hundreds on jobsites. I think if they could handle professional use they would be more ubiquitous, batteries are really expensive at full retail. I think most people just wait for genuine batteries to go on sale, or steal them.
There is also the danger of recycled cells, but I am sure that can be mitigated by buying "quality" fakes.