I'm a .45 ACP w/ 230 gr. HP ammo sort of guy myself. However, the 5.7 I have is very light, easy to hide and, above all is something I can shoot well. If I decide I want to carry something light, I'd far rather have a 5.7 FN than any 2 inch class gun.
I see the 5.7 FN as one of the choices for those that choose to carry something minimally effective ballistically, but alot easier to carry comfortably than a full size 9mm - .45 ACP. E