To tell you the truth they don't really work for me. The program modes have a habit of getting confused and flashing random warning LEDs when it should be firing the shutter. I think they are probably just a gimmick for the benefit of the marketing types and sales people.
I much prefer the greater control of the aperture priority "A" mode. Silhouettes can be made by changing the exposure compensation (+/-), with the obligatory bracketing.
In fact, if I remember rightly, modes like "Portrait" just set the lens to a certain preset aperture, f/4 I think it was. "Landscape" was set to f/8.