Yeah, I installed one last fall. The biggest advantage is on those mornings where its foggy right around freezing, and as you drive the fog freezes to your windshield.
Also, it will quickly clear a light coating of frozen rain or a skiff of snow, once it's up to temp.
The Rostra heater will bring the water temp up a few degrees in just 30 to 45 seconds. In 2 minutes, it's something like 10 or 20 degrees warmer. Of course, only the water in the little black reservoir is heated, but that's usually enough to do the job.
I put a toggle switch on mine to shut it off when I don't need it. I think it has a thermostat (or am I thinking of the heated wiper blades... ). In any case, I thought it was a good thing. My only problem is/was I couldn't find a convenient place to mount the reservoir. So it's sort of sitting in amonst the steel brake lines by the PDC.
FWIW: I also did the heated blades and a few years ago did the heated seat. They all work well.
-Jay