I have 5 JPs (2 JP15, 3 GMR15) and have no complaints about their products or customer service. After building my own using mostly V Seven, Criterion, Geissele, and JP parts, however, I will not buy another factory rifle. Don’t build to save money (you most likely won’t), do so to truly understand how the firearm operates.
if you don’t see yourself taking the gun apart to trouble shoot, I suggest you just buy a complete JP (nothing wrong with trusting the experts when it comes to firearms).