OK, now we're talking about things in my field.
I've played with all the above mentioned firearms. Matter of fact I was working with a few P90s, FiveseveN pistols, G18Cs, MP5 PDWs, ect.
In my opinion . . . . the P90 is overly bulky and cumbersome for it's intended purpose. Add to that the point that Jonas brought up about questions about the rounds actual street performance due to lees then carbine like velocities. (Note, it has been used in one campaign successfully, in Peru)
If the round was suspect in the P90, you can imagine what it's like in the "FiveseveN" pistol. I will say that the little sucker is a dream to shoot. Minimal muzzle flip and recoil.
As has been mentioned in part, the ammunition for the P90 is extremely restricted being "Armor Piercing Handgun Ammunition", meaning it's about unobtainable. A popular gun writer buddy of mine mentioned in passing that the 5.7x28 ammo is $4 a round currently or $200 a magazine for the P90 or about $4K a case, IF you could buy it. I don't know if this is accurate, but I have no reason to doubt him. (The last two years at SWAT Roundup, the FN guys not only had to account to ATF for all the live rounds, but all spent cases as well)
The G18 is the select fire Glock pistol, they also make a ported version called the G18C. Now those are a blast. Talk about fun to run. It's even pretty controllable when used with a stock. I actually enjoy firing the G18 since they improved the selector switch so it doesn't rap my freakin knuckles at 1500rpm.
There are a couple of other PDW platforms on the horizon. Most will not be available to the everyday man, but I've gotten wind of at least one (using 50 year old technology) that should be pretty interesting.
John