I have owned 2 MPX's. if you do not do VERY regular maintenance on the gas system you will find yourself throwing the guns across the range because you just lost a match due to feed issues from the gas system not properly cycling the BCG.
I can tell you from first hand experience that it is very frustrating to have that happen to you. IF you have the time and effort to keep up with the maintenance then the MPX is a great platform.
I sold both of my MPX's and went and bought two JP GMR-15's, changed out the 5 stainless weights to 3 tungsten and 2 stainless and the guns shoot very flat and group way better than the MPX's ever did. People asked why I did not wait for the JP-5. I did not want to wait and only be able to afford one gun. I prefer to live by the mindset of 2 is 1 and 1 is none. I know the JP's are very reliable and are very forgiving maintenance wise but even with that, I prefer to have a complete back up. This allows me to shoot them both and they each behave identical and that also extends the maintenance cycle.
I have owned a DB9R that I heavily modified and also a PSA PCC. sold them both to get the sigs. I wish I had bought the JP's earlier.