"Un bel avion est un avion qui vole bien" - Marcel Dassault
Up until recently, I'd gone with Dassault's blurb that a beautiful plane makes for a good plane.
Aesthetic abominations like the F117 have destroyed that faith however. In real life that thing looks like a low-budget prop for a bad scifi movie. Mind you, it did get retired rather quickly so maybe there is some truth left to this saying.