Yah, that's what I am wondering about as well... ammo.
One of the regulars at my range (who is also on this board, won't out him) has both the PS90 and the MP7 and he ... ahem.... virtually
never brings them to the range. Why? His cover story is that he doesn't want too many people that he doesn't know constantly asking him about them, asking to try, etc. and that is very legitimate.
The real answer (I think) though is ammo. MP7 (4.6x30), extremely difficult to get any ammo, and the price tag might make you wish you didn't find any

. He has been telling me for months that he has some on the way. Still waiting. 5.7x28 is not a heck of a lot better I think. There were some people on here who were going to try and reload some using .224 bullets, and maybe cut/neck some casings down. Not sure where that went really. Can you even get dies from 5.7x28?
If the Shooting Edge has it in bulk, that is great. I didn't know that. Maybe Redleg can give us an answer. All I know is that, here in Vancouver, I have almost never seen the 5.7x28 around. That is, at the shops, at the ranges, in the possession of friends who own guns that shoot it.
I see also that a couple of the guys that bought up the PS90's put adds in the EE looking to buy 5.7x28 in bulk. Not sure how that is working out for them.
The PS90 is a cool toy, no doubt about that. A 50 round mag pinned to 5 is a bit comical, but it is an awesome toy nonetheless. Regrettably I think it is a toy that you only bring out a couple times a year. Not much to feed it with.
Is it just me or does the FN FS2000 magwell seem deeper than on other rifles, meaning that the insertion and removal of LAR-15 mags might be tricky? What a deal breaker. The SIG 550 - 556 hybrid seems to be in my opinion a much better deal.
A guy at my range had one for awhile. He was using the LAR-15 10 rounders with the Dlask mag-coupler. It worked well. Could get a little tricky trying to stick the little mag in there on its own. I've never tried that though. Only ever used it with two 10-rounders back-to-back. So anyway... my point is... the 10-rounders are fine, you just need to use the mag-coupler.
The thing about the FS2000, sure it is cool. So unique. It fires the brass out the front. You feel like you are in the "Aliens" movie when you hold it. It seems pretty accurate, but the trigger is horrendous in my view. It's like a big plastic block that moves straight back. Having said that, however, if you want something different, and you can shell out upwards of $4K for a unique rifle, why not?