[jtodd]

I've done a good number of FWH only beers and calculating the bitterness as a 20 minute addition would have been flat out wrong.
..and no I didn't do it both ways and do hundreds of blind three way tastings. I don't need too.

I've brewed many FWH beers, and I would have to agree with Malticulous. The bitterness is smoother and more refined than a normal 60 minute addition, but the perceived bitterness is way more than what a 20 minute addition would give you. I'd say it's probably more like a 45 minute addition. Also, I have never sensed any hop flavor or aroma coming from FWH. Just a smoother bitterness. I think the belief that FWH somehow adds hop flavor or aroma is one of the big fallacies of FWH. At least, that's how I perceive it. And you know what they say, perception is reality.

[tygo]

I just tapped an all FWH ESB yesterday.  First time I've done it with just FWH and no flavoring addition.  There's definitely hop flavor there although it's different than what you'd get by adding in a flavor addition.  I guess I'd tentatively describe the flavor as more subtle but it's there and it's good.  I've only had a couple pints and the kegs still settling a bit so more testing is in order.