I've run about 10,000 rounds through my MBX Extreme PCC, and while it's pretty good for the money out of the box, the recoil and construction causes the gun to break itself apart. It's been extremely accurate, and very well balanced, but over time the gun basically has torn itself to part. I've gone through 2 buffer tubes because the castle nut has ripped through the staking and shreded the buffer tube threads, and the front handguard has also stripped the barrel nut, and won't maintain a consistent position relative to the frame.
Once I shot my friend's Gen 3 MPX, I was sold. Very minimal dot movement, and the recoil is extremely soft, even with overpowered factory ammo running 150 pf. Follow-up shots and shooting on the move is much easier, and you can get away with much less technique with high accuracy.
Reliability-wise, it depends on what generation you're running for the MPX. Talking to some of the top PCC shooters like Max Leograndis, Brian Nelson, and Zack Smith, the earlier Gen 1 and Gen 2 models were really hit or miss; I've got about 2000 rounds through my Gen 3, and it's been pretty outstanding. Even talked to Keith Garcia a little about it during the Safariland match, and he also swore by the MPX vs. JP GMR.
Just my $0.02.