While I'm not a fan of the ENVI 5 GUI, I have had to use it just this once so that I can directly open my Landsat data via the MTL file. The MTL format changed in early 2013 so ENVI classic will not open it directly anymore. You can still open the individual TIF layers though, but it's just not great when you want to retain information from the MTL, such as radiometric calibration coefficients.