Modular Tangible User Interfaces (TUIs) - i.e., UIs made of small-scale physical modules-offer novel opportunities for tangible interaction thanks to their highly customizable form factor. Such modular TUIs were proposed with different shape of modules and bonding strength between them. The problem we address in this paper is the lack of knowledge of how bonding strength and shape of the modules impact usability. We present the first study exploring the impact of bonding strength and module shape on subjective user ratings when interacting with a magnetic modular prototype. We assessed three levels of bonding strength (low, mid, high) and two shapes (cubes and rounded cubes) in a controlled user study. Participants performed eight common manipulations found in the literature for (non-)modular TUIs. Experimental results showed that (1) cubic modules are overall easier and more satisfying to manipulate, except for precision and bending tasks, (2) low strength impairs UI solidity, but high strength impairs precision tasks with cubic modules.