h.bp119660 wrote a review of CAMP USA Ball Nut on December 30, 2012
Familiarity: I've used it several times
Good gear for small cracks.
I used the number 2 to protect the crux of a 24 (low 5.12 in US terms I think) we were working. I bought them specifically as no other piece would fit for the crux. The only other piece below that (which we could get into the crack) was a number 1 RP. We took about 5 falls on the ball nut and it held every time.
However, we did notice that the ball nut shifted about a bit when we were dogging the move, even with a draw on, making the placement a bit sketchy and requiring a reset. It could be operater error, but other climbers I have talked to who also use ball nuts have noticed the same thing. So, I would be particularly careful with ball nuts to check they have not moved if you are resting on them repeatedly.
I think there is room for improvement of the design of these units. For example, the plastic stem is thicker than the size of the head on the smaller nuts, which can make it difficult to set the piece deep in smaller cracks.
Overall, the bottom line is ball nuts do a great job of protecting very small cracks and, for me anyway, did what no other piece could do. But, maybe more than other types of pro, care needs to be taken if they are subject to lots of movement.