The FiveseveN has one use - dropping JBTs as they kick in your door.
This is why they don't sell them to civies and the ammo is classified "Agency Only" by the ATF... they won't even sell these to individual agents or officers, but only to agencies.
If you want to see how the FiveseveN and P-90 handle in a simulated environment, and you own a copy of Unreal Tournament, go download
Infiltration
Although, personaly I prefer the HK MP5/40A3 to the P-90 (but both are cool with a surpressor).
You can legaly buy the FourtyNine, but keep in mind the thing has a 10lb trigger!