Good body armour should really feel like a second skin and protect the parts that it was designed to protect. The Forcefield Arm Tubes use some pretty swish technology to try to achieve this, but it does seem to falter on some fundamental issues.
The removable Armourflex protectors within the pad will take an impact effectively and cushion most scuffs and blows well, but to achieve ultimate protection, the pads need to continuously remain in their desired position, which doesn't always happen unfortunately.
The Lycra sleeve and strap on each Tube definitely help with security but also contribute to your arms overheating. The lack of a hinge between the elbow and forearm section seems to push the pad down during extreme flexing of the elbow - a common and vital movement required in mountain biking - exposing previously protected parts, which is exactly what you don't need while you're hacking down a mountain.