Works poorly here in Florida. They treat my windshield as a complimentary option at my local Toyota Dealership when I get the cars serviced. It lasts about 2 days in the Florida Sun. I think the heat evaporates the hydrophobic compounds. I actually see more "spotting" on my windshield after they treat the car. Probably because they're not cleaning the windshield properly before applying. I've done applications of it at home and it's no better.
That's strange, the carrier (which smells like alcohol) evaporate but silicon doesn't, the only way it comes off is by wearing. I've used it for years and it works perfectly.
Did you follow the instruction exactly? I've seen people who just spray it on and expect it to work. You need to clean the windshield, wipe it on evenly with a rag, wait till dry, then buff till you can't see any swirls only clean windshield.