I have a few JP guns.
As much as I hate to say it, my experience with them has been mixed, and is based wholly on who I dealt with over there.
John (the owner) is a good guy. He had one employee I wouldn't pee on to put out a fire. That fellow is now long gone.
The CTR 02 is a top of the line 3 gun rifle. They give a sub MOA guarantee and usually the rifles beat that by a fair amount. Once I got it working right ( no thanks to the one employee) I have not had a hitch with it, except when I made some bad handloads. I have a LMT, a barrett, and a couple colts which are in the same reliability range, but they did not have the toothing difficulties that I did with the JP- but the JP is all I use in 3 gun.
The second has a heavy barrel on it and is a .5 MOA to 300 yards rifle.
They do know how to make an accurate upper. Their barrels are good.
The muzzle break/comp they use the most now is a Bennie Cooley style comp that is legal for the more restrictive 3 gun divisions. That works pretty well, especially with a heavy barrel. I'm able to see the prairie dog vaporize in the scope with a shot. Their old muzzle break ( I think they call it a recoil eliminator) works better, but puts you in open division automatically in the 3 gun game.
They were one of the first to come out with an adjustable gas block. The ones I have work great, but I have a couple other guns that came with other company's that work as well.
The bolt carriers come in 3 weights. The full mass carrier is for reliability. Its essentially a milspec type carrier. They make a midweight carrier (which is steel and somewhat cut down) and a lightweight one that is made of aluminum and skeletonized somewhat. If you tune your load to the light carrier and your gun, you essentially get rid of the sensation of movement of the bolt when shooting.
The JP trigger is a single stage trigger which will give you a really nice light trigger pull. Not a good choice for a military gun or one that has full auto capability. It will not have take up when properly set up, so its not a good choice on a fighting gun as you may not realize you have the pressure on it, and it will fire, but for a range gun its great. I cannot shoot mine with gloves on as they will fire before I'm ready.
They sell a special hammer and springs for separate installation- the only caution I would give is that the light springs and light hammer will give a sub 1 pound trigger pull that breaks like glass, but may well not work with the harder (CCI 41 or 43) or milsurp primers. My rifles do not like milsurp ammo because of that.
I think JP still also has some proprietary sighting systems, but in my opinion, unless you are going for 3 gun shooting, regular stuff works well enough for that.
The other thing that JP does really well is the fit and finish of the guns is superior to pretty much every other AR manufacturer I've seen.
I'm not into building AR's. I buy them "pre made" so I cannot give you comments on their suitability as far as components for mix and match, but as a system, its very nice (assuming you don't try and make it do something out of its intended scope).