I think the trick is to purchased them after 3 years of age or so ... that way you can see if the "glitter" is going to hold or not. Most ginrin loose that "sparkle" as they grow and as the scale widens/gets larger the little facets (sp) which allows the light to glantz off starts to spread or flatten out and the light reflection just isn't there anymore.
I don't care for doits either, but that's just me. Although I do have one.