Colloidal crystals assembled from stimuli-sensitive hydrogel particles composed largely of the thermoresponsive polymer poly(N-isopropylacrylamide) display unusual phase behavior because of the inherent "softness" of their interaction potentials as well as the particle thermoresponsivity. In this contribution, we review results from our group that illustrate the use of such soft-sphere building blocks in the construction of colloidal crystals. First, we describe the utility of temperature-induced volume phase transitions in controlling the crystallization and melting of the colloidal crystals. These methods then enable the study of the complex phase behavior of certain types of microgels. For example, it is proposed that multiple weak attractive interactions between particles can drive crystallization at particle concentrations well below the hard-sphere freezing point. Finally, the utility of soft-sphere crystals in the development of new photonic materials is presented in examples of laser direct writing and photopatterning of colloidal crystals based on a photothermally directed crystallization method.