What is claimed is that it adds some flavour and a softer bitterness, although with a higher IBU in comparison with a 60 minute addition.
What he said. Some references say that you can estimate FWH by assuming its a 60 min addition plus a 20 min addition in one dose.
Agreed - to a point. IME FWH utilize small amounts of hops more efficiently because you can extract 15 min addition-type flavor from a bittering hop. But you cant just move all your flavoring additions to FWH.
I use FWH mostly to reduce total hop mass in the kettle without sacrificing bitterness or flavor. I've changed my IPA recipes, taking a portion of my higher alpha hops from the 15-20 min addition and adding to FWH, normally in place of the 60 min addition. I'll use a small amount of super high alpha hops at 60 min if I can't get the desired IBU with 1/2-1/3 of the flavoring hops as FWH.
Do a side-by-side with a Pale/IPA. You can never have enough of it around the house anyway.