I was in high school the first time these things were popular; back then, had a bunch of cars with them in the fall, and the numbers dropped off precipitously with the first couple snows. Between sand and salt getting kicked up, plus bumping over snow and ice, they didn't last at all. I imagine the modern LED based ones would survive better, but given how well headlights do in the spray around here, they're going to look like sludge unless you're under there cleaning them off every 10 miles or so.

If you're going to do it, do it in the spring and do the install in a way you can get them off in the fall if you want them for more than a season.