I just got a FAC Al flattop receiver and a German barrel/trunion assembly. I've always wanted a G3K-type rifle, and the ability to mount an Aimpoint directly to the top of the receiver really appeals to me. I'm planning on using high-quality parts - no SW or Hesse shit, and I know the receiver is an unknown quantity at this point.
I have a Federal receiver G3 rifle. Never once jammed on me. It already has a semi-flat top for a scope ring so it was ready to go.
The G3 has it ALL over the FN/FAL since there's nothing to adjust, just load & shoot.