Unsurprisingly for a motocross based firm, Fox have always made good protective gear.
The problem with their standard guards was that the articulated top (knee) section was unstrapped and therefore free to flap about unless you wore long pants over the top. The new Fox System setup uses the same asymmetric hard plate over 'biofoam' padding armour, with twin calf straps, but you get a long neoprene and nylon knee 'gasket' included. This pulls on like a long footless sock under the armour, then you fold the top back over the armour to hold it in place over the knee. It takes a bit to get the fit fettled, but once they're on we were really impressed.
There was very little slippage even on two hour jump and jive XC rides, and no obvious pressure points, strap cramp or mobility restriction.
We've not had a big off in them yet, but past experience says they protect knees well and at £30 they're a steal compared to separate pad and sleeve combos.