I've beaten the Atari 2600 Ghostbusters many times. It's actually one of my favorite games for the Atari. Of course, I'm a HUGE Ghostbusters fan, so I guess I'm a little biased when it comes to it.
Using the ghost bait to stop Mr. Stay Puft isn't all that hard. When you're on the map screen and you notice your Ghostbuster logo freeze up and the roamers (ghosts) start to quickly move towards the Zuul building, you have to immediately flip the right difficulty switch.
Getting past the Marshmallow Man at the end is indeed tough. He jumps back and forth and you have to use perfect timing to get your Ghostbuster to run under him without him touching you. If you can get 2 busters past him, you get a screen of them standing in front of a buiding, crossing the streams.
Not as cool as the C64 ending of Ghostbusters but it sure beats the NES ending.
Later!