Just dance was probably delayed because Sony asked them to delay it to make it a "newer" game closer to the holidays (at least on their system), and also so it didn't compete with Everybody Dance.
As for Everybody Dance, it will probably do okay, especialy if it has a Move bundle with it this holiday. On a personal note, I already have the game. My wife won some party thing for it that came with Move cups (they change color when they get cold 
) and party favors. Anyway, it's not too bad. I do wish it had more than 40 songs, though, but I suppose more will come as DLC. My wife likes it. It also has a 20 person party mode. Haven't actually tried it yet, so I don't know how that works. I guess it just track the massive blob behind you. 
