Took me a little bit of tweaking over a couple of days to get the battery to last as long as the Hero, but it's there now.
Initially it was shocking, I was losing 10% per hour on standby
And HTC have been a bit tricky by altering the sync settings on the desire compared to the Hero, but I finally managed to figure it out.
I'm sure most already know this, but in case you don't here goes:
First thing
menu > settings > Wieless and networks
Scroll down to Mobile networks selection and uncheck 'Enable always on mobile'
Next
menu > settings > Accounts & sync
Background data and Auto-sync are both checkboxes. You can turn these off (uncheck) and this means that nothing will autoupdate, you'll have to do everything manually. This will save some battery but I find it a bit of a pain. I've got them both on, we can alter the settings in the apps themselves to lengthen battery life.
Below that you'll see 'Manage accounts' There should, by default, be your Google mail account, stocks, and weather. Turn off stocks. Unless you're particularly interested in keeping bang up to date with constant stock updates then you really won't need this. Turn off weather too if you're not bothered about that. I have weather on, but once again we can alter the weather settings elsewhere. Leave your Google mail sync on...unless you really don't use it at all.
then
menu > settings > Sound and Display
There are three checkboxes I unticked. These are: 'Audible touch tones' 'Audible Selection' and 'Haptic Feedback' If you really want these feedback options on then do so but it will impact battery. You can also turn these feedback options off for the virtual keyboard via the keyboard settings menu (open the keyboard and hit the little cog wheel icon)
The next thing to do is to manage app updates. You can do this by opening up the app and pressing the menu button. There will usually be a settings option in which you can manage update intervals.
For example the weather widget.
Click on the widget so that it opens full screen.
Select menu > settings > update schedule.
You can now select how often the weather app will automatically update, although this will be unavailable if you turned off automatic weather updates in the previous 'Manage accounts' section.
I've selected an update every hour. I can't imagine anyone wanting more than that unless you're a real weather update freak.
One of the biggest culprits for accessing data and sucking the life out of your battery is the news widget. I've ditched it altogether, and noticed a big improvement.
If you do want to use it then I suggest you go with as few feeds as possible. To do this, press on the top bar of the widget. You'll see '+ Add Feed' but if you hit menu > settings' then you'll also be able to delete feeds. I've only got one, in case I ever decide to use the widget, and that's the BBC. Your choice is up to you.
If you select the 'settings' option you'll also be able to select the update frequency. I have it on Manual, you can set it up for various times, but obviously the more frequently it updates the more battery is being used.
Task Killers
These are a bit controversial on here and aren't strictly speaking essential, but if you know what you're doing with one they can be very useful for letting you know exactly what apps are currently running and, therefore, giving you a good idea of what may be trying to communicate and what is using your battery.
Using the above settings, or at least something similar. I now lose about 1-3% of battery life per hour on standby, which is pretty good. Of course if you spend all day playing youtube videos and streaming music then you'll get far less, but what do you expect