Paul, thanks much for opening this forum.
I'm an 80/20 convert. This stuff is easy to put together and change around to fit your needs. Granted, not the cheapest material you'll find around but it's worth the price.
Someone asked on another posting how flat or linear are the 8020 extrusions. As a real world application but keeping in mind that my X and Y axes rails are only 24" in length, I tested the frame I put together for my CNC machine (Please welcome... "Contraption 1") and found it to be flat to <.003". I'm sure I can tweak a bit more precision out it by just adjusting the frame but don't have a testing method that's any more accurate than that. Furthermore, I'd think that any imperfection/warping in the aluminum frame would be corrected by the steel rail when properly bolted on.