In civillian legal form, there ain't much going for it other than the arms it is chambered in. The pistol and matching P90 are sexy guns, but the ballistics are not much better than a .22 magnum. Ammo costs twice as much at least, and you ain't buying it at WallyWorld.
5.7x28mm "Cop Killer" Cartridge Myth
"A comparison of the performance of the .22 Winchester Magnum Rimfire (WMR), introduced in 1959, and the 5.7x28mm is interesting. The .22 WMR shares the same bullet diameter (.224") as the 5.7x28mm cartridge and their muzzle and energy figures are actually rather similar.
Winchester's Supreme .22 WMR loaded with a 34 grain jacketed hollow point bullet has a muzzle velocity of 2120 fps and muzzle energy of 339 ft. lbs. from a (presumably 24") rifle barrel and a muzzle velocity of 1690 fps with 216 ft. lbs. of muzzle energy from a 6.5" pistol barrel. (That is just a hair over 20% velocity loss.)
Winchester's Super-X .22 WMR 40 grain jacketed hollow point (JHP) and full metal jacket (FMJ) bullets--Winchester offers both--have a MV of 1910 fps with ME of 324 ft. lbs. from a rifle barrel. Fired from a 6.5" pistol barrel the same loads register 1480 fps MV and 195 ft. lbs. ME. Note that rifle ballistics of this load are nearly identical to the 40 grain 5.7x28mm factory load as fired from a sub-machinegun length barrel and from a pistol barrel the 5.7x28mm is only a puny 21 ft. lbs. of kinetic energy ahead of the .22 WMR. I have never heard that semi-automatic .22 WMR squirrel and jackrabbit rifles were considered a particular threat to police, nor have I ever heard the .22 WMR described as a "cop killer" handgun round. Actually, the .22 WMR is considered a "mouse gun" caliber by most handgun authorities.
Frankly, I know of few individuals who would be willing to spend more than $900.00 for the FN Five-seveN pistol and then shell out more than $20 for a 50 round box of ammunition. (.22 WMR ammo commonly sells for $7-$8 for a 50 round box.) I imagine that the Five-seveN probably also fails to spark much interest with criminals, as these pistols are rare and thus almost impossible to acquire except from legitimate, FFL dealers. Typically, the criminal element usually relies on the availability of larger (9x19mm, for instance), more effective calibers that can be easily acquired in the stolen gun market.
This leaves us with the obvious conclusion that the 5.7x28mm cartridge has little advantage over the .22 WMR in terms of killing power and serious disadvantages in terms of the price and availability of both firearms and ammunition. Without doubt, being shot with either can have deadly results, but there are much greater threats and more important things with which to be concerned."