I doubt that DJ Hero was a "horrible" game, in fact I wouldn't be surprised if it was at least decent. But it just doesn't seem like my kind of game, I don't really have that taste in music, but who knows.
Regardless if DJ Hero 2 is a good game or not, I will not buy it due to it being published by Activision. I will not support Acti-Kotick for any reason, I don't care if DJ Hero is a godsend to music games or whatever, I just cannot support Activision. I may be missing out on something grand, but that's my stance on this.
However, that won't stop me from potentially playing it (borrow it if a friend gets it or something).