If you read the lastest NAW, Adam Hayes says it is one big key to his success. One look at that dude's wall tells you that listening to anything that works for him is a good idea!
That said, there are several free apps that basically give you the same info as the Moon Guide. I use the Mossy Oak Hunting Weather app and it has an Solunar option. If you know the overhead/underfoot times, you can correlate those to the peak activity times (first/last two hours of daylight) and look for the days that you should be hunting. I don't think the Moon Guide itself is a gimmick, but you can get the same info for free.