The Five Seven by FN Herstal is a unique gun. The cartridge used is what makes it most unique as it is rarely used. The 5.7x28mm is an interesting choice as it's more powerful than a .22 but less than a .223.
The stye of the gun is nice although some (myself included) find on the ugly side. The double stacked magazine will hold up 20 rounds in a standard size. You won't find another hand gun with that capacity unless you are taking about the extended magazines which stick out 7" from the bottom of the gun.

So what exactly is wrong with the Five Seven?

Well, FN doesn't make junk. So you can expect a reliable and well built pistol. The biggest hurdle is the price and availability of ammo. You won't find any 5.7x28mm at your local Walmart. Local gun stores/chains will be hit or miss but expect to pay around $.50/round if you can find it. For that price I'd rather shoot 5.56 or .45 ACP...

There are threaded barrels available if you want to run suppressed. However, the 5.7x28mm is terrible round for suppression. It's loaded supersonic so unless you can make custom ammo, you won't be able to find any subsonic rounds for it.

While the 5.7x28mm is available in armor piercing designation, the use of them in a pistol is illegal. Some indoor ranges will not allow you to fire the standard 5.7x28mm rounds on a pistol range because of their high velocity/penetration potential.

Last, the gun is priced around $1100. There are many other quality guns in standard calibers for $1100. This makes the FN Five-Seven more of a novelty gun. To each their own, but I would personally buy a .50 Desert Eagle if I was shopping for a novelty gun. Food for thought.