I always get IE first, but I make sure to take AS through Zeal or something. AS is particularly important on the ADCs with CC, like Ashe or TF. BF is for an additional lane sustain, but why get it? By the time you do, the laming phase is closing, and your DPS now matters more than sustain. Maybe if you are Caitlyn versus a poke comp or kill lane, but IE's passive is crucial to that DPS.
Oh, and I love going double Phantom Dancers. 100% critical hits crate the same DPS as BF, and with IE and attack speed, it beats BF.