Okay, I've had my Desire HD since Beginning of November, so 3 months. I previously had a Sony Ericsson Xperia X1i, which I found to be a very nice phone, with some "challenges", which for someone like me, is a good thing.
The Desire HD was a big step for me because I've only ever owned Windows Mobile as a phone OS, and I have to say I was impressed with Android. The way it handles applications is amazing and the community and Apps available for it are huge in quantity as well because it's such a popular phone. I love the large screen but I wish you could get one with a 4.3" screen and a full Qwerty keypad, (The Z has a 3.6"screen).
I am not going to go into a massive amount of detail here because you didn't come here to read a phone review (I hope lol), but overall the Desire HD is faster, smoother and nicer to use than any phone I've used, which includes an iPhone 3GS and iPhone 4.
I would certainly recommend this phone to anyone.