Reactivity, yes - works the same as MobX's autorun. But not derived values. You can ensure something gets updated by wrapping it in $:
$: value = $x + 2
but you can do that somewhere else as well
let y = 20
$value += y
Makes it insanely hard to track where changes come from. You can't say "this value is _only_ updated by these changes".
You can do this with derived stores, though. But each store must be it's own, separate value - with MobX's observable you have a central object representing your entire state, each of which is made automatically reactive, and putting in a derived store is just prepending with -get-. It's much cleaner.