Here is a picture comparing a front-side detail to a box stock impeller. Both are the same American Turbine casting.
As Steve pointed out, the blades are thinned and asymmetrically sharpened, dulled only enough so to prevent getting cut while handling. The shape of the blade surface is recontoured to reduce a low-pressure area. The transition from the shaft to the hub is worked to improve quantity and quality of flow in that area. The shroud, near the wear ring, at the impeller inlet eye is thinned and trued to smooth the transition of flow into the impeller in this area. All of this is done reduce cavitation. The result that the end-user experiences is, a harder bite with the most dramatic affects being on the launch.
A good working front detail job will make the impeller "act" like it is one full cut size larger. Basically, it makes the impeller work more efficient by reducing cavitation. The pump will pull the engine down harder, approximately 200 RPM. This usually corresponds with an adjustment to the cut size to allow the engine to operate at peak power.
Notice, that at no point did I mention polishing or surface finish. In a jet boat, polishing makes absolutely no difference whatsoever. If we had access to the instrumentation that NASA has for testing liquid rocket engines, we might be able to measure the slight difference. A good detail is all about the shape and not how pretty it looks.
I will post up pictures of a back-side detail, where the trailing edges of the blades are foiled, later
