Many people, many opinions. I will just say this. I now have or have owned the following 9mm PCC's Just Right Carbine, TNW Aero Survival Rifle, Beretta CX-4, Kel-Tech Sub9, Sub2k and Sub2k Gen 2. Marlin Camp, High Point 995 and 995 TS, RRA 9mm AR, CMMG 9mm AR, and something else I forgot...umm.
The Just Right Carbine (JRC) is one fine piece of work. I have the later model. I skipped the first generation as they worked out the normal bugs. I have had mine for a few years now and it is a reliable and accurate a 9mm carbine as I have owned. I like the fact it uses AR FCG;s and I have "tuned" mine for a few bucks by just changing some springs. Everyone who shoots it likes it. All the people I know who have one of the later ones like them. So, its a good piece. That said, its not the lightest of my group. It is, however, one of the smoothest shooting and most accurate.
I have an TNW ASR (Aero Survival Rifle). Its reliable and shoots fine. It is lighter than the JRC by a little. It shoots fine. I have the two stage trigger in mine due to personal preference. The trigger is OK, but not great. Its not well made as the parts fit loosely on their shafts and the trigger itself is narrow and doesn't have a good feel to it. If you want a lighter carbine it is a good pick. There are some issues however. Some have had problems. Some have had to be sent back to TNW. Mine went back for some TLC before it began running properly. Another issue is the quick change barrel. It can tend to work loose while shooting. It doesn't fall off but accuracy drops as you would expect. They have a "fix" in that you put a new hand guard on it. You fix the barrel coming loose but lose the ability to quickly beak down the rifle and fit it in a small bag or backpack.
The CX-4 is a Beretta. Chrome lined barrel, 100% reliable, very compact and light. Hard to find and not cheap. Trigger is not great but there is a DIY trigger job that improves it.
The Kel-Tech should not be compared to the others. Its a special piece, made to be very light and fold up. Its more like a long barreled pistol than a PCC. Its not the nicest to shoot as its design has limits. Shoot one once and you will see. It has a special place however as it is so small and light and can be folded up and put away or brought out to action quickly.
The High Point works, period. Its not pretty. Its not a slick, precision German or Italian gun. Its American assembly line bottom dollar priced piece that will work now and tomorrow. Its cheap in price and you know it when you hold it. But it goes BANG every time and hits the target.
The 9mm AR is just that. Cold took a rifle made to be a gas operated 5.56 rifle and converted it into a blow-back 9mm PCC. Newer 9mm AR looking rifles are modifications of the original Colt design, some with improvements. Some even use Glock magazines. I sold both my 9mm AR's. I had them running fine but the JRC would just out shoot them. The JRC was smoother. The Beretta CX-4 was lighter and smaller and would out shoot them. The TNW ASR was smaller and lighter and quick to break down and stow away. For me, the 9mm AR no longer had any advantage. For others it is popular. You have to decide what suits your needs and desires. Luckily you have many great choices now, so much better than just a few years back.