Got mine today and have to admit: I fell in love immediately.
To answer JohnnyEgo’s question:
Yes, it totally responds to different sounds and exactly resembles the lower frequencies of whatever you do. Your bass has a snappy envelope? You will FEEL the snappy envelope. Your bass has a slow attack and long release? You will FEEL that, too.
Of course the module does NOT just rumble “randomly”, what would be the point?
I turned the volume of the OP-Z all the way down and pushed it against my ear and could hear my bass-line only through the rumble-module just the way it should sound.
But in general it does not work like an audible subwoofer (if you do not push it against your ear), meaning: Using the internal speaker together with the module does not really change the sound of the OP-Z much for me (unless putting it on a really resonating surface). But using headphones together with the module changes A LOT for me. The bass feels full and juicy an I love it.
If bass is lacking in your track, you will of course not feel much. What I found a bit strange:
When placing the OP-Z on a solid table, the rumbling gets almost lost (probably “absorbed” by the table). It’s best to hold it in your hands or place it on your lap.
Also important, the OP-Z does not clatter in any way, while using the module. It’s rocksolid.