From my recollections (and the Schwinn Heritage board), the Tenax tubing used by Schwinn in the mid-late 80's was essentially SP guage main tubes without the surface finish, as alanbikehouston said. The stays and fork were Tange. So these frames weren't the bottom of the barrel steel.

The few I've ridden (Supersports, Tempos, LeTours) with the Tenax seem to be solid frames, and tend to have some nice details like bottle cage reinforcements, stainless(?) dropout faces, etc. I tend to keep my eye out for them since one can find them at garage sales and thrift shops for next to nothing.