Sorry to get off-topic but the magazine for the P90 is pretty inovative (once they fixed it). It puts more weight on the top of the weapon to help reduce muzzle climb if your using full auto, the round has good penatrating qualities as well.
I can't get to it now because Google is down, but there used to be a picture illustrating the different combat loads, one for an M16/M4, the other for the P90, while using the same number of overall magazines, you get more ammo to send downrange (duh).
However, the P90 was never meant to replace a service rifle, only those used by say, tanker and helicopter crews. Perhaps a police agency here and there as well.
As far as the FN five seven goes, I have no personal expirence with it, it is supposed to function really well, and seems to be pretty accurate. I've read it has good shock statistics. Also remember it is not limited to 7+1, 10+1 mags either. I've only seen one shoot once at my range, but I have always wanted to test one myself.