Meaning of "auf geht's" and "los geht's"
Yes they do mean "let's go" and you can answer "Willst du ins Kino gehen?" with "Los geht's". But "Ja gerne" would be the more common answer.
I heard this expression in a song of the same title by die Fanta Vier and Los geht' from Ramsi Aliani's song ' Schlagalarm'. I thought they all meant ' Let's go'.
Well, there's a big difference between language in music and actually spoken language.
*"Los geht's" is more common than "Auf geht's"* I doubt it. Probably depends on audience, region, etc.