It's a nasty round when it hits, that is for sure. Low weight on a elongated projectile makes it an instant tumbling round, and the speed it goes at increases the number of tumbles a lot more than 5.56x45.
As far as I know they are designed for Urban combat. Maximum penetration on soft targets, minimal on hard targets so they don't go through walls and injure non-targets. They were also designed for low weight/high capacity so more ammo could be carried and the 50 round magazines wouldn't be...
I just look forward to the day that more than two manufacturers make the ammo. Since reloading it is rolling the dice due to common failures at the neck of the case, it would be nice to see a little competition to get the price to stay at a reasonable level.
I don't know if some of you guys on here frequent Armslist as well, but I've noticed a bizarre rash of fake FN 5.7 Pistol listings up there recently. They always include a stock pistol with upwards of 500 rounds for $1100-$1200, and are always flagged as scams within a few hours. What's up with...