Block copolymer micelles in the size range between 10 and 100 nm are investigated as model systems for soft spheres. The zero shear viscosity eta(0) and complex modulus G* of micellar solutions are studied via dynamic mechanical spectroscopy and shear viscosity measurements over a wide range of concentrations. Depending on their structure, block copolymer micelles exhibit the characteristic rheological behavior of hard spheres, soft spheres, or polymers. With increasing concentration, hard-sphere and most soft-sphere samples exhibit a sharp liquid-solid transition as apparent by a divergence of zero shear viscosity eta(0) and the development of a frequency-independent elastic modulus. The transition occurs at a certain volume fraction which can be related to the softness of the particles. In the solid regime the elastic modulus G’ exhibits a characteristic concentration dependence which is related to the spatial variation of the soft sphere repulsive potential. We observe a G’proportional to Z(1.48)r(-2.46) relation between modulus, aggregation number Z and intermicellar distance r which is close to the theoretical prediction G’ similar to Z(3/2)r(-2) Of Witten and Pincus derived for polymerically stabilized colloidal particles.