I've owned this phone for a few months now, so I feel I'm in a good position to write a review on it. First of all, I work in IT, so I have a fair idea of the phone market and made a considered choice when I purchased the Desire HD.
The Desire HD has, for the most part, been an excellent phone. For the average phone user it will more than exceed their expectations. The phone fits comfortably in the hand, calls are clear and crisp, and the screen can be seen in reasonable UK sunlight.
The key feature for me is the camera- a nice, clear 8MP lens with a decent LED flash produces perfectly acceptable photographs for any event. Registering a Google account on the phone will also back up your photos and contacts to the Cloud, so no more fears about lost or stolen phones.
HTC also bundle some nice features, such as the ability to locate, wipe and ring the phone from their website. Just make sure that you register for the free service!
One thing that is missing is a front facing camera- something that will become more and more useful with the new features coming in the next version of Android- Ice Cream Sandwich, scheduled for launch on the Desire HD in coming months.


Tip:
Although some phone companies provide "signal boost" apps, switching these off (along with Bluetooth and wireless when you don't need them) will lengthen your battery life.