My car has one; useless. Its covered by road grime or slush or a raindrop all the time. I can see out of it for 10 minutes after leaving the car wash and that's it.
Mine (A MB GLK) works very well. It's never been too dirty to see, and it has a great overlay showing range to objects and current trajectory.
Maybe this summer it will be better, but its been crap all fall, winter and now spring (in Iowa).