I own and love a Desire HD. I have played around with the iPhone 4. After a few weeks of use you get a feel of the great stuff, the aweful stuff and the minor niggles.
Great:
* Locations, downloadable maps etc
* Contact list with integration with everything (Facebook,Twitter, email, sms,phone, etc) under a single contact and auto photodownload from Facebook. Simply awesome and better than the iPhone app as far as I've seen.
* Single inbox
* Multiple Scene sets
Not so great
* Speaker volume is pretty low
* Phone speaker volume is ok but not great in noisy surroundings
* Battery life sucks
* Gmail inbox is not integrated with central inbox
* Car navigation offers free badly spoken Google navigation, free unspoken Route 66 navigation and paid spoken Route 66 navigation. But it only offers Google and premium from the carpanel, not free unspoken.
* Calendar Agenda view can't be put on a page as a widget
* Folders don't show small icons of what's in them like on the iPhone. They don't seem very smart either. An auto close type feature would be nice.
Niggles
* Photo Album widget is a bit slow and unresponsive sometimes
* The apps list doesn't slide page-by-page like on the iPhone but continuous. Given the screen size that makes the amount of items on screen a bit overwhelming
* Not possible to modify RSS addresses
* Translator app doesn't allow switching from/to language
I think the screen of the iPhone 4 is much better than that of the HD, but on its own, the HD screen looks pretty impressive. So AFAIC it's a question of good enough.
For me the ultimate test for this phone was: does it make my life easier and does it do as promised. Yes in spades! Especially the way in which GPS integration is used in a lot of apps to offer me choices of what's near (public transport, food, parkings etc), saves a lot of time and even makes working on a phone at times faster than on a PC, which was a big surprise to me.