I have been using EzGrader now for a little over a month.  During that time I have applied it to well over 200 US stamps, including some difficult ones with non-rectangular designs.  I have been very happy with my purchase.
I have found EzGrader to be especially helpful when evaluating stamps that I buy online.  On HipStamp and eBay, I use it to evaluate the centering of the same stamp offered by multiple sellers.  Often I am surprised by what I would have selected using the naked eye vs. what EzGrader tells me is the better-centered stamp.  EzGrader has saved me many hundreds of dollars, and led me to choose better quality stamps.