I have one, bought from Extracta, have used it only a couple of time but with reasonable results on both occasions.
Both times were on patterned carpets with reasonable pile that had cigarette burns.
I clean the carpet as normal, identify a centre reference point and cut out the damaged section, this is the worst part, when you see a big gaping hole in the middle of their carpet!! I then remove a slightly larger (+2-3mm) section from a spare or hidden piece of carpet, it needs to be slightly bigger due to the fact that you lose some of the tufts from the edges when you are cutting and lifting.
Check for size, dress up with scissors or scalpel as necessary, apply some adhesive spray glue to the underlay and the back of the new section, fit and match. Brush pile to hide seams.
When I was practicing I had various levels of success, I found it nigh on impossible to get an acceptable result if the carpet itself is not in a reasonably good condition. It's a doddle doing it on new samples of carpet, but that's not the real world.
Always quantify with the customer beforehand that perfect results cannot be guaranteed.