What Brad said.
I have found the program to be absolutely invaluable for selecting components for very specific applications such as subsonic loads for AR-15 variants. Need to know what powder will cycle a 10.5" .458 Socom pistol with a 500 grain cast bullet and remain below 1050 fps? No problem, there only three powders which will do it though and how long would that have taken by trial and error? Make a prediction, find a starting load, compare case volume/bullet weight/expansion ratio to published data as a sanity check, and run some test loads over a chronograph to make sure you're safe to proceed and work on up.