First and foremost - heavily hyped beers rarely live up to the hype, but Heady Topper is definitely one of the best commercial IPA's I've ever tried.
Aroma is like burying your face in a bag of hops. There's plenty of fruit, but there's also this great hop cone/lupulin on your fingers smell that rarely makes it all the way into the finished beer. The flavor has some citrus, but is more skewed to the piny/herbal side. Bitterness is nice and firm on the middle of the tongue, but not harsh or abrasive. The hop aroma and flavor are clearly more the focus than the bitterness. The finish has plenty of hop flavor continuing on for a good while.
What I really like about Heady Topper is that the bitterness doesn't feel harsh or raw, and it doesn't feel like someone just painted hop resin on your tongue that's never going to wash off. As much as I enjoy beers like "Go To" and "Enjoy By", or my own IPA's for that matter, I get some bitterness character that's a bit rough (almost like raw hops) when trying to push the hop flavor envelope. Whatever they're doing at Alchemist, they really have it dialed in.