Let's be honest, Cheech & Chong weren't exactly artists, but some of their earlier movies were a lot of fun. This was their 4th or 5th movie, and it's basically a tired rehash of the same stoner jokes they were knocking out in Up In Smoke. Having said that, it is still fun to a certain audiance; the same audiance who like Beavis & Butthead or Jay & Silent Bob really.
It's not just the infantile humour, it's that age old story of 'loser makes good' that we all find so satisfying over and over again. While C&C don't really achieve anything in any of their movies, they do have fun not achieving it.
In this movie they end up in Amsterdam, where they are mistaken for Burt Reynolds and Dolly Parton (obviously). You can pretty much imagine the rest of the movie as is. It's stupid, it's childish, it's juvenile - and that's exactly the point.