It is neat. When it works. However bug splatter and snow tend to render it useless. Not so much a design flaw, just impossible to prevent stuff from covering up the sensor.
They are in path of wipers so window in front of them is kept clean. Rain did not seem to effect it. The lenses are about 1 inch in diameter and wide aperture a likely do not even "see" window in focal range.
Cool, the company I work for generally has put them in the front bumper, so it gets splattered with everything. The only complaints I've had with them is not that the radar cruise doesn't work, but when there is a malfunction with the sensor, the cruise can't be used like the regular old cruise we are all used to, it just doesn't work.