As I can judge, DGX650/660 are very successful on the market and well-priced. The initial DGX-620 (there also was DGX-520 with lighter keyboard) had only 32 voices of polyphony, which seems ridiculous for a such big keyboard, but as Stephen says, he enjoyed it anyway.

As I see it, Yamaha targets people who don't want to adjust a lot of things. While we are used to keyboards like PSR, we often forget how frightening it can look from outside dealing with all these menus, mixers, tabs and so on.

The crucial thing to me is an ability to mute parts of an accompaniment and DGX doesn't have it. As I believe, too many parts playing together is something which makes an arranger to sound cheap.