When I first saw the cartridge I had my doubts about its efficacy. When I first saw the P90's magazine arrangement I had my doubts about its reliability. When the first series of tests came in it confirmed my previous fears about its performance.
...And then after having the chance to train with a group of people who have more operational experience with the P90 than any other LE entity in the United States and who have been in MULTIPLE firefights with the P90 and seeing them give the weapon a big thumbs down because the bad guys were saying "Stop that!" when you shoot them with it....
Well....I formed a less than favorable opinion about the weapon and the 5.7 cartridge.
As a toy, it is a fairly neat (though very expensive) one. As a tactical tool, there are much better options out there.