The fact that they are waiting to release a demo until the game was finished is fine. At least it won't be "different" than the final product like so many other demos (which can come across misleading sometimes). I'm still gonna wait on a demo before considering to purchase the full version, just like I do with all my game purchases now.
Anywho, I think it will be interesting to see what has comes from the initial employee stand against management to get DooM 3 to be the next project worked on by id. Ya'll do remember that don't ya? :}
From what has been released of D3 so far (not including the leaked material which I haven't seen anway), I think the game "looks" cool, but, I have yet to see a real "reason" to say I must buy this game. I'm at a point where the "eye candy" is nice, but it's NOT the main reason I play games...it's the gameplay (always has been, always will be). If a game is pretty but lacks in the gameplay department, it's kinda like a great looking car without a decent engine to get ya anywhere fast.