They're solid brass, and the screw-mounts are fully secured. They're not about to fall out or off or get easily pulled out from the leather. I wouldn't recommend them for any kind of actual fighting use- more because the spikes will do just as much damage to your hands as to whatever you're punching. But beyond that, yes- the're as solidy affixed as it's possible for me to make them.
Looks great, but not an effective design. When making a fist and punching, those metal studs will transfer the impact force into your proximal philages (finger bones) and will hurt, maybe even fracture them. The metal studs would be more effective if they were attached to a solid frame that extends between the fingers, transfering the impact to a bar or grip in the palm area (see Brass Knuckles). Or the metal studs could be solidly attached to the back of the gauntlets, in line with your metacarpals but extending beyond the knuckles of your fist to take the impact and transfer the force to the whole of the gauntlet and strappings.
Also, you should never hit with your fingers, you hit with the knuckles; and they are designed as an accesory, so its vital for it to not hindrance the movement of the fingers, like a brass knuckle would