I still haven't got to grips with the battery, its not lasting a whole day
Other than that i likes it
I get about 30 hours of battery out of my Desire; that's with phone calls, three emails (work, uni, gmail), twitter, etc., but wifi is off most of the time. Wifi is a big battery killer as is bluetooth, so turn those off if you're not using them.
Also, the 2.2 official update includes a new radio software which improves battery life significantly. Get the official 2.2 update from HTC, or if you've got the skills root and flash the latest radio onto whatever ROM you're running.