It's OK for getting a rough idea, but then again if you are in an area with no decent definition aerial shots, it is no better than a compass. A lot of people here use dishpointer.com (I believe they designed the original software) and others download the swedish Microwave calculator from smw.se (I also have the PDA version for my Ipaq).
At the end of the day, you still need to align the dish using the signal from the satellite, whether it be using a small satellite meter, or the meter on your receiver.