Costs the same as Youmuu's, and yet due to the passive offers a MUCH more massive powerspike; extremely cost efficient.
Youmuu's is a great rush if you need to roam rather than desire the burst from DB, but Duskblade's spike is clearly beyond that of just "1 full item" and can often turn a matchup you've already played poorly into one you're totally dominant in; nullifying even major leads of the opposition.
I understand it helps deal with ADCs, but it was argued by Riot that it would be given a niche alongside Youmuu's so the two were debatable choices as a first-rush-item; but DB is **by far** the winner of the two. It also offers the *invisible power* that Riot so strongly preached against having as part of their game design.
P.S. I love assassins; hands-down my favorite class, but Duskblade is sucking some serious fun and variety out of AD Assassin itemization. It's honestly a total snooze-fest to see every AD and their mother build this item.