User blog:Krufix/Last Whisper - When to get?

I've heard a lot of arguments about when you should be getting Last Whisper on an AD carry.

Some people insist that it's an optional item to combat people who've stacked multiple armor items and that it's not for every build.

Other's have said get it 3rd-4th after IE, Bloodthirster and Phantom Dancer.

However I have so far found that this is the item I get when I want to melt faces into a puddle on the ground. I can say from my limited experience that I can have IE, BT, 2 PDs and a Guardian Angel, and deal no damage into late game even against squishy targets. However I can add in a Last whisper at any point in the game and THAT is when my power surges and never drops off.

I remember reading a post which did the math and found out that on a champion with just IE and against a champion with just 30 armor, a last whisper as your second item gives more DPS increase than any other item in the game.

I'm unsure about the validity of this however.

So I have to ask, when do you build Last Whisper? Is it a good idea to get as your 2nd item after BT or IE?