Ai, Rico is right, they do tend to lean forward a tad when crouching.
Also, each velcro strap thing seems to have a slightly different length to them and each pocket has slight variences so the front pocket willl usually hold the mag better than the back one.
One thing to note however is you need to move the strap all the way to the back where its a little bit wobblie. It takes a while to setup but its doable. After that the mags come out nice and smoothly. Unlike G36 mags....
Also the P90 mags are so long that they actually interfere with the other pistol mag pockets and it actually makes it somewhat difficult at times to shoulder your gun. See that US patch on the picture you have attached? that'll be completely covered by the mags. And as usual the side pockets will be hard to reach. but unless you're trying to put a mag in it, it's not too bad.
Overall it works. You'd be hard pressed to find other vests that hold p90 mags and the ones we got were a pretty good buy. But its by no means desgined specifically for p90 mags, so there are a bit of quarky problems with it. You do feel very protected by your mags though, its like armour plating.
The good thing about it is it's very adjustable, you can pretty much fit any kind of mag (cept for maybe the AK) without too many problems and fit tightly. We'll be at the FR game this weekend if you wanna check it out.