I'm mostly git -> mercurial -> sapling -> jj. For me, Mercurial -> Sapling was a mostly lateral, slightly upward move. Sapling -> jj felt really good, though oddly not because anything felt horribly wrong or missing in Sapling. Things just feel nicer and lighter in jj. Once I know a couple of things, I can combine them to do lots and lots of other things without learning any new commands. In fact, the total command set is pretty small, yet covers about the same range as other systems (a little more here, a little less there). Also, improvements come in at understandable and predictable places: at its current stage of development, it's not growing brand new commands. It is instead slightly improving existing commands to enable significant new capabilities. It has the feel of a well-designed system with mostly orthogonal components. (Give it a decade, and perhaps it'll start crumbling under its own weight too...)