5.7x28 was designed as a caliber that could be used in a pdw or sub machine gun as well as a pistol.
It is an excellent military round but not really good for self defense. The ammo available for it now IS NOT the round you would want. Initially when it came out there were black tip AP rounds available. It was soon discovered these rounds would penetrate body armor at distances out to 200yds. The us govt politely asked the manufacturers not to import any more of it. Hence these rounds if found will command a high premium.
The rounds available now to civilians will simply pass right thru most targets transferring almost zero energy into them. For self defense nothing works better than a nice sized chunk of copper covered lead.
It was hoped than troops could carry the Fn P90 pdw and the Five Seven so one caliber could fuel both guns but they have since moved onto other things. It's an extremely accurate round but I wouldn't carry it. Not to mention the guns themselves are pretty large.
Lots of controversy surrounds the gun and caliber:
FN 5.7×28mm - Wikipedia, the free encyclopedia
It is the gun used in the Ft Hood shooting.