Nick has reminded me that I hate endnotes with a passion. However, this hatred aside, I do believe that endnotes are the standard for publishers trying to bring more scholarly works to the mainstream. Whether this is useful - I don't know. I am sure publishers would have done some sort of research into the issue. (I lie, Nick, the publishers union has my family hostage and are forcing me to write this.)
That aside, one of the worst cases of endnotes for me was reading Martin Hengel's Four Gospels and the One Gospel of Jesus Christ. The book is 1/3 endnotes - rather interesting endnotes. Endnotes that had me constantly flipping through. It really did annoy me - I just want to stay on one page, not jump ahead, read the next 10 footnotes to avoid turning back, going back to the main text, forgetting the footnotes I previously attempted to remember, etc. You get the point, not nice!
Another odd case of endnotes was with Ben Witherington's The Gospel Code. There were no numbers or markers that there was going to be a reference, you would just turn to the endnote section hoping to see an exert of the sentence with a note after it. It was really odd and confusing!