I definitely recommend that anyone getting hand cramps give the add-on an extended play trial of atleast 30-45 minutes to see if you notice a difference.
The added bulk on the right and the little extra bulk on the left removed about 80% of hand fatigue during a 45 minute MK7 play session and that's only because the left side isn't as comfortable as the right due to the right having that added bulk that the left lacks.
I have no idea how comfortable the add on will be for games that are actually intended to use it though as I don't have a game to test it out with. But just simulating playing a dual analog game feels comfortable enough. The only awkward part is the double shoulder buttons, but that's mostly because I'm not a PS gamer and have also never used the Classic Controller Pro either. But it feels alright. The one thing that could make it better would be some handles to grip, but over all, it feels much better than a bare 3DS in hands.