The trouble with Sto-Fen type diffusers which cover the flash head, is that the flash is still coming from a small, highly concentrated source and you still get the ugly hard shadows on walls etc. They may be slightly diffused, but there's not much in it. They will though, somewhat reduce the impact of the harsh flash on skin tones.
By far the best thing to do with flash is bounce it from a wall or ceiling for a much softer more natural light without the ugly shadows, but of course it isn't always possible. For wedding interiors I finally came up with a lightweight flash umbrella as being the best solution, but good results can be obtained using any portable white reflective surface.