I've had SampleTron for quite awhile and have never found a use for it. It would be just the ticket if you were specifically emulating an early-70's prog rock sound, something I've never undertaken.
Funny, back in the 70's I absolutely loved the sound of a Mellotron in songs like "In the Court of the Crimson King", but today it mostly just grates on my ears (except in that particular song, which has aged well). I've gotten far more mileage out of Martinic's free
Vox Continental. He also has a
Farfisa, but I haven't used it yet.