I have one of these Surplus Arms and Ammo lowers I got when a friend was ordering some from AIM. Here's my observations:
We ordered a total of (3) to try em out. They seem to have a little less fit/finish on the machine work. The area where the top of your grip goes, for instance, isn't nearly as clean of machine work as it transitions into the buffer tube area compared to a Spike's and DPMS lower side by side. All the lowers went together fine, but one of the lowers seemed to have a little too much material on the front of the lower, where your front take-down pin goes in. This was only on (1) of them, so it apparently made it through QC. This space was hitting on the quadrail, so we had to hit it with a dremel to remove a little material to get the clearence needed, as it was making removal of takedown pins a bear with the pressure it was putting on the pins.
Overall, they seem like good lowers. Although, IMO, I've seen a little better QC on the Spike's/DPMS.