I've been using Invisible Glass for 3 years and love the results. But if I have to deal with a smoker's car, I've found that rubbing alcohol will cut through that film rather quickly.
word Invisible glass is the best, i also like to use newspaper to clean all the bugs and gunk on the outside of the windshield along with my invisible glass, then do it again with papertowel. afterwards it comes out looking like a brand new windshield!