Hi - I have had my Desire HD for almost a fortnight, and have had it on charge at least daily (mostly twice daily).
I don't have many apps running, or excess syncing, and I always switch the screen off when I'm not using it. I hardly make any calls or use the web, and I'm always in areas of good reception. WiFi is off most of the time. GPS is always off.
What surprises me is the amount of battery the phone uses when I'm not actually using it. I leave it on Airplane mode overnight, and in 8:23 hours last night the battery dropped 20-30%. My brother's iPhone only drops 4% under the same circumstances.
Looking at what's using up they battery, I can't really understand why it runs out so quickly - Cell standby - 35%
Phone Idle - 32%
Android system - 19%
Display - 6%
Voice calls - 4%
WiFi - 2%
com.android.htccontacts - 2%
If anyone has any ideas why my phone's eating up battery so quickly with so little use, I'd be very glad to hear them!