For what it is worth, I'll throw in my two cents.
My FFL lent me a couple FNs couple weeks ago, one of them the Five-seveN. If someone wants to carry it, I get it. If not, I don't.
First off, I appreciate everything on this thread that has been said. I find the ballistics of the 5.7 round, the history of the firearm and the mechanics very interesting. I won't even pretend to comment in the level of detail some of you are. Maybe this can serve as the common person's perspective.
Here are the comments I provided to my FFL who has been trying to sell me one for some time:
"We went to Cabelas for ammo for this before hitting the range. There was an army guy from down south in the ammo row who had a 5 7 and was talking about a foot of flames when he fired his and how it was novel, but sat in his safe a lot. We picked up some American Eagle ammo. No foot of flames. Maybe it is different than the FN ammo... It shot really nicely. Good groups, no muzzle flip, back on target fast, very easy to control. Interesting controls and break down. Controls weren't' right where I would have liked and the slide release was harder to work with than need be. Understanding what it was made for, it was very intriguing. It wasn't one to fall in love with for me though. Considering the size, 20 rounds is kind of unimpressive too.
Right or wrong, looks (plastic all over) and round type of the 5 7 remind me of the PMR. Except the PMR is waaaay more fun. The boom and fire of the magnum, 30 rounds, ridiculously light weight. You just laugh manically when you shoot the PMR. Maybe if I had MilSpec ammo for the 5 7 I would laugh maniacally for that too..."
I have other firearms I prefer to carry, so I know the Five-seveN would not make it out often. The cost of the firearm and the ammo compared to the PMR means that's a lot of money to sit in my safe for something I am only intrigued with and not in love with.