I know that Quadangles are one of the more popular old school BMX's and I know the design of the thing was pretty innovative but as for the build quality I don't think it's up there with any of the big hitters like GT, Skyway or Hutch.
I have a Quad and the welds on it aren't bad, but in places are pretty average I have to say. Bikes built in the US command a much higher sale price but from the evidence aren't necessarily any better build quality in my opinion.
The best welds I ever saw had to be on either the Skyway TA or Redlines Mk 11's, which were both mint.