Even though it's mounted a bit back from the windshield in the pic, the curve in the larger unmounted image makes me think it'd fit right above the windshield, where the curve is. Hence I think it'd really screw up aerodynamics, putting a huge object right at a spot where wind moves fast at highway speeds. So besides the cheap price, it'd probably cause a loss in MPG, and likely increase wind noise.
All in all, I'd suggest getting a deal on some brand name lights and mounting them to the factory crossbars.