Thin films are a central component in micro- and nano-systems. However, thin films are typically metastable, and will undergo solid-state substrate dewetting even at moderate temperatures during processing. Dewetting can be problematic or helpful, depending on the application, but the geometry of the dewetting process can vary widely, producing many different final configurations. We present a model that predicts shape evolution during dewetting of films, including features such as the rate of film edge retraction, the formation of ridges and valleys, pinch-off (a valley becoming so deep that it splits the film), faceting, and hole formation. Metal films that evolve by surface diffusion typically have at least partially facetted surfaces. Our simulation method derives from that of the fully faceted technique by Carter, Roosen, Cahn, and Taylor [Acta Met. et Mat., 1995]. The model can begin with any starting shape and symmetry, and evolves the shape over time towards the equilibrium configuration. Though any two-dimensional system can be studied, of particular interest are long, narrow films (lines). To verify the applicability of the model, it is compared with experimental results from a study on dewetting of patterned Ni thin-films on MgO substrates. Lines of single-crystal Ni were patterned with varied widths and crystallographic orientations relative to the substrate. The lines were then annealed to study the edge retraction rate and minimum aspect ratio required for pinch-off as a function of orientation. Comparisons of the model and experiments will be discussed, as well new models to address three-dimensional dewetting phenomena.