Are these receivers any good?  How do these compare to say a factory FMP or a JLD PTR91 receiver?

The receiver metal is stamped by FMP, them welded here in the states (=USA made receiver).

Just as good as a FMP receiver since they are stamped using HK demention forming stamps.

i have one , it hasnt been built on yet, but the quality seems great, bolt carrier slides in and out real nice, rear site installed easy, mag well and rear buttstock fit  were a little tight but i fixed that real easy, the only complaints ive heardabout the 91 receiver  are from people who have had bad dealings with Special Weapons in general and tend to bash their products because of it.

I had Jayson @ IGF build a CETME kit on one for me.  I was very pleased with the outcome.  The welding on the receiver was very nice.  I think they use a plasma welder on them.  I wouldn't hesitate to get one.