I've had them for at least 4 years. I installed them right-side-up, which I regret. They work better if you install them upside down as B-rock did (or so I've heard).
The driver side one (which gets the most use) is no longer working properly. It worked fine for the first year or so, but since then if the temperature is below ~30°F (which is when you really need it), it won't turn on. Very annoying.
Ryan