I think Sense UI, with its 7 home screens, sweet integrated widgets, and (slightly better) ease of use is what makes this phone comparable, UI-wise, to the iPhone.
Sense UI is part of what I'm proud of when I'm showing off my phone.
Home is great too. Android is a great OS. But 3 home screens are the same as 7. And you have to hunt for all the widgets you want. Most people find what they want. But 7 screens gives you lots of real estate to put all of those widgets. Most of your apps have to either be in folders or using Application Manager or just under the button at the bottom of the screen.
Most of my useful apps are right on my 7 home screens, so that anyone I can toss my phone to can play with it (without understanding Android).
Home requires you to understand Android a little bit. Which is great for me. But I've converted 7 people to the HTC Eris/HTC Hero already (depending on their carrier). Sense UI is part of what makes that possible. It's a little slower, yes. Laggy, yes.
But sleek and smooth looking. Yes and yes.