I have found it to be quite helpful.
I have found it to be very close to chronograph data.
Would I want to “max out” a load based on QL? Nope.
Being able to see what happens when we change seating depth is a big help. Giving a start point for a given powder/bullet/cartridge combo is a big plus. Start reasonable, load a few, and see how close it compares to chronograph. Move up from there.
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.
I like it. I wish I had sprung for it sooner.
It has its short falls... straight walled cases. But I refer to it regularly. I even have been checking my own known data and never once have I found it to be considerable different either with pressure or velocity.
Once you have made adjustments to get predictions to equal your chronograph results you are good to go. There is an app that gives optimal barrel time nodes for your barrel length and QL gives you barrel time for each load you predict. Adjust your load to match one of the OBT nodes and you have a good starting place.
I was on QL for couple hours yesterday. If ya read my hair brained idea in my 350 legend thread.. I was researching powders and loads to move a 56g bullet in the legend most efficiently. It showed many powders one would first choose like H110/296/2400/#9/lilgun... all did well but Power Pistol took the pride for lower pressure and higher velocities. Knowing QL is iffy with straight wall I stopped at 50k and then would drop back a couple grains an work up. But quite confident Ill be safe. W/O QL I would have had no clue.