I have an Incra iBox.

I also have a D4R and in a moment of weakness... maybe spurred on by the vapours of some C₂H₆O... I purchased their F3M jig.  The instructions, like all their other manuals are helpful, however, after the initial test run after purchase, it has hardly seen daylight.
For me, finger/box joints are for quick/dirty jobs and if I chose drag out the D4R, I finish up going for the dovetail. When I truly want a quick dirty, I'll go the Incra

I won't say it's the worst item I've bough for a long time, however, it's down the list of my better moves.

If your first cut is too short... Take the second cut from the longer end... LBD