Well, they aren't really. The glass isn't the same as a true ellisoid. I have the "Depo" style lights, and actually if you take them apart, the bulb does not even sit the way a normal bulb would, it is perpendicular. Light output is not great, but that's what fogs are meant for.

I wanted bright light and didn't really care for fog lighting, so I installed HID Xenon bulbs in them and they light up a little better, but still not good enough as far as brightness.