Hello all, this is my first post to these forums.
My experience is a little unusual. I had seen DD advertised before but never picked it up, and the main reason why I purchased BD was the offer of recieving DD for free with it. Two games for the price of one, who can beat that? Nevermind the fact that my copy of BD didn't come with DD - I was so pleased with BD that I later went and purchased the first game.
I played BD entirely through Act 1, and about halfway through Act 2 by my reckoning before DD arrived in the mail and I installed it.
Interestingly enough, I am now completely hooked on DD, and haven't had any desire to play BD since I started DD.
In my opinion, the plotline of DD seems far superior, as is the fundamental playability. Having to switch between two characters wasn't much of an issue for me in BD, and the skill tree is not confusing, although it can be irritating at times. One thing, though... I have to play BD with the volume off because the voices seem overdone and cliche, and downright irritates me - especially the Death Knight's. His voice makes me cringe. No offense intended, simply opinion.
BD is still a good game, and worth the money, but I have to agree that DD has a much higher 'fun' and 'enjoyment' factor, which is crucial to any game.