1. The ammo it's really designed for, designated SS-190, is not for sale to civilians in the U.S., being "armor-piercing". HERE's an interesting test of that capability. The stuff you buy in a store, the blue-tipped SS-197, doesn't pierce armor but it also doesn't run the velocities its cousin does.
2. The pistol was essentially an afterthought, the 5.7 originally being designed for the fully automatic FN P90 submachine gun. I'd love to have one but, you know... That platform has a barrel long enough to produce the promised velocity. The Pistol, not by quite a bit.
3. The range I shoot at the most offers free lane time for anyone who will let them chrono their Five seveN, so being a cheapskate and curious too I did that. Mine chrono'ed in the 1500-1700's for everything I've got in the way of factory ammo. Not one actually exceeded 2000 fps.
4. The round is a bugger to reload - it's really small, which means the tolerances on powder are super-critical given the relatively high chamber pressures involved - about 50,000 psi versus 30,000 for the 9mm. You cannot use a case polisher - they're dull-looking from the factory because there's a lacquer coating that facilitates feeding. I do NOT reload them myself.
So all that makes the gun too much of a pain in the butt to own, right? Wrong! It's a total hoot. It's just expensive (for me) because the ammo has to be just so. But it's a flat-shooting, low-recoil, loud-blast sweetie in your hand. YMMV... ;-)
Unfortunately, almost all commentators go off on the will-it/won’t-it question of performance against armor. I don’t think that’s the important question.
While ss190 is interesting its ability to defeat body armor from a pistol, I’m more interested in terminal ballistics. Leaving aside armor penetration, the ss190 is ballistic ally equivalent to ss192, 195, and 198 against unarmored targets. All three of those loads are available to lesser and greater degrees, for quite reasonable prices. All three chrono at just under or just over the magic 2000 fps mark.
So the question remains, what kind of efficacy can a civilian expect when using the non-sporting rounds for self defense?