Its only worthwhile if you can get the AP ammo loaded to full spec. The US loads are basically equivalent to 22 magnum w/ no hardened penetrator bullets allowed, except for M&P sales. Case life for reloading will be short, as the shoulder is blown way forwards upon firing from all weapons tried so far...
Asides from the mag capacity, I'm really not impressed w/ either the P90 or the Five seveN pistol. If the P90 was in a different caliber, it would be great, its much more ergonomic than the F2000 cheap AUG knockoff.
The pathetic terrorist who recently used a Five-seveN pistol made most of his kills on wounded personnel that didn't escape being head shot. Other reports of the round's terminal ballistics are generally not favorable. Its only real advantages are light recoil and armor penetration as compared to other handgun rounds.
Not that I pretend to be any great expert on the round: I've shot some and read more, that's it. My opinion is worth what you paid!