I have used both car wax and car sealant on my windshield in the past with excellent results. You just have to make sure the windshield is SUPER clean to start with. I usually apply wax on to the clean windshield in a crosshatch pattern followed by a circular pattern, then remove and buff multiple times with a clean, high-quality microfiber cloth to minimize any potential hazing, etc..
Works very well, overall.
Currently, I have Gtechnic G1 on my windshield, and while it works decently well, I'm not sure it works that much better for the cost than an occasional application of wax. It certainly lasts longer for a single application than the wax does.