I've used a few of them. I love them for a fixed stock (bonus that you can quickly convert back to adjustable). There are a lot of variances with receiver extensions. The cross screw prevents the stock from sliding off, but is dependent on the tube "ramp" (where the adjustability bar starts) being in an exact spot. If you have your extension screwed in more or less will affect it (puts the ramp further fore or aft, thus allowing the stocks cross screw more room for play). On all of the ones I've used, the plastic front cap does not sit flush on the end plate (I've actually had to file down some of the "nub" on the front piece to make it fit better), and if you sling from the stock, it CAN pull the stock rearward a bit (until the cross screw hits the "ramp" on the tube).
Hey, it's a fixed stock on an adjustable stock receiver extension. It's not the best choice for a fixed stock (there are REAL fixed stocks for that)... but it's benefit is that it's quick to convert back to adjustable (I shoot both in PA and NJ, and it's nice to be able to put the FCS on for NJ use, then swap back when I get back to PA).
Think of it as a temporary, nice, fixed stock.