The title gives you the gist of what we're talking about here.
This is our new best friend. Previously, it had a short cast time which made it bad for our rotations to sneak it in. NOW though, we have it as an instant cast, and let me tell you, I love it.
As a frost mage, at level 81 in my Live gear, it's worth some DPS.
On a target dummy, using a basic rotation not including FFO, I do around 5K in beta. On live, I do around 4800. Now, considering that this is without a full compliment of useful glyphs, that's not terrible. The talent overhauls certainly didn't HURT my DPS.
I threw FFO in, and did a consistent 5500. That means FFO is worth 500 DPS, 10%.
Start planning rotations around FFO / FO, because, yes, it's that good.