I've had this phone for just under a year now and use it a lot, for everything from taking snaps on the frankly brilliant camera, using internet browser and apps, using the GPS to get around all over the world and occasionally making phone calls and texts!
First things first, this is a big phone. Its also kind of weighty, but not in a brick kind of way just when you first get it it seems gigantic. But, as I've had it so long it seems perfectly weighted to me and the big screen... I don't know if I could do without it now! I have watched countless films on the phone and find it perfect for this, with a lovely crisp HD display and a decent built in player with dolby. The camera as I mentioned briefly is also excellent with a decent zoom and pics taken in 8mp. The pics often come out better than on my deidcated samsung 12mp camera! Even in low light the pics come out well although thats when you start to need a dedicated camera really...
The touchscreen is responsive and apps and webpages loads quickly (if you've got a good 3g or wifi connection). A plus for the big screen is that sending a text or email is less fiddly than on iPhone for example. Also, the fact that text fits the screen when you zoom in is another plus over iPhone.
The Android app store (Google play as its now called) does fall some way short of the apple store though and a lot of the best apps for apple aren't available or are poor imitations. Saying that, there are still loads of great apps for all your needs whatever they may be, check the website if you need to know before you buy.
Oh and its a phone too and a very good one at that. I've had a few calls dropped and the odd glitchy conversation when it shouldn't have been, but I have used several networks since owning this phone and would say that its better as a phone than my previous handset, an iPhone 3gs which dropped calls regularly and occasionally wouldn't even dial out with full signal.
A few other pluses quickly: you can up the memory with removable SD cards to 32gb, you can put any format video or music on and it'll play it, you can also plug and play/drag and drop - no need for stupid itunes to put music/films on or remove pics etc. You can use the phone as an internet provider if you're on the move, simply plug in the USB and switch on the internet pass thru/wifi hotspot and if you need to unlock your phone for use with another network, its pretty straightforward (unlike iPhone).
For the battery, yes it can be poor but its a smartphone it depends how much you use it! I have got over 24hrs out of the phone by switching off 3g/wif when not in use and not playing games all the time etc. At worst its down to around 5 hrs if you watch a film, make a few calls, browse the net and play a game for a while. I bought a spare battery when I went travelling (about £3 on ebay for an 1600mAh) and that has been a life saver on long bus journeys.
Its a bit older now but I would still say that the desire HD is a strong contender for best smartphone available. If you aren't too keen on being told what you can or can't do by apple for example then you'll be very happy with the possibilities the desire HD offers.