I just filled up today with some of the pink Rain-X stuff. Don't know if/how well it works yet...

I've used the Prestone before (last winter) and it works.

Not only do they not freeze (I don't think normal windhsield washer fluid does either) it helps to de-ice your windshield upon application. What I have seen is the normal fluid hit your windshield and freeze up... But not in the tank, AFAIK.