Dad likes it too and he does all the spraying.  He's been using it since the 90s I guess and he likes what he knows works and hasn't let him down.  Retailers push the liquid and price AMS around $18-20 per 51# bag.  I can see how liquid would be easier to handle and store but has always been more expensive as well.  We've found it in the $12-15 range the last couple years so hopefully we can still find it going forward.