The reason that we were looking at getting the 5.7 x 28mm was because right now, 5.7 is cheaper than .45cal.
wat
As stated already, the PS90 is fucking worthless as a semi-auto. The 5.7x28 is nothing but a glorified .22 magnum. The gun is not accurate at range, can't be fired prone (comfortably), and the round is grossly underpowered for... anything.
As a fun to shoot gun, yea it handles great and it'd be great for plinking... if the ammo wasn't so damn expensive. The fact that the ammo is rare and exclusive is also an issue in a SHTF situation.
The way I see it, the short comings of the round itself and the insane price of both the Five-seven and the PS90 are NOT made up for by the weapons themselves. They're novelty guns. If you're looking for something to practice with, plink in the woods, carry, kill zombies, you have an infinite number of better choices.
For the record, if I had the money I'd buy a PS90 in a second, but that's because I've got my SHTF weapons covered already.