Has anyone built up a gun on the frame and slides?  I'm thinking of building up a midsize 1911a1 with all GI parts except for the RRA frame and slide.  The complete guns look nice too, how do they compare to Springfield Armory's 1911A1's?

I don't have one, but I've handled several.  These were the DCM (hardball) match versions.  They were works of art.  They came in with slightly light triggers and I got to bend a few sear springs to bring them up to the match spec.  

Beautiful workmanship.  The owners liked tham, too.

Kings Gun Works, a custom gun builder which specializes in 1911s prefers to build on a RRA slide and frame. Kings is one of the premier builders of 1911s.

Unfortunately they can no longer supply the RRA slide and frame due to Caliban rules. But they can build on any slide and frame the customer supplies.

I have a Springfield and my friend has a RRA, both built by Kings. The Springfield frame is a little rougher than the RRA so if you want beauty you'll work a little longer on the Springfield but as for function they are both about the same.

Since you are going to use "all GI parts" and not build a custom "match" gun I'd use whichever one I found cheaper.