There has been little discussion on how the materials used to create deformable devices, and the subsequent interactions, might influence user performance and preference. In this paper we evaluated how the stiffness and required deformation extent (bending up and down bimanually) of mobile phone-shaped deformable devices influenced how precisely participants were able to move to and maintain target extents of deformation (bend). Given the inherent haptic feedback available from deforming devices (over rigid devices), we also compared performance with, and without, external visual feedback. User perception and preference regarding the different devices were also elicited. Results show that, while device stiffness did not significantly affect task performance, user comfort and preferences were strongly in favour of softer materials (0.45 N·m/rad) and moderate amounts of deformation. Removing external visual feedback led to less precise user input, but inaccuracy remained low enough to suggest non-visual interaction with deformable devices is feasible.