I'm overall pretty happy with the battery life I get out of my Desire. If I'm just using it for calls, texts, some web surfing, ect - I can squeeze several days out of the battery. If I'm playing a lot of games (Angry Bird addict here, I know, I know!) then I find myself plugging the phone in to recharge on a daily basis - which is to be expected.
Also, as others mentioned, GPS really drains the battery. Went out for a scenic drive the other day to a nearyby town. Battery was at about 70%. I was snapping pictures like crazy, noticed a bit of battery drain. But then turned on GPS for directions. After about half an hour, I was in need of a charger.
By comparison: my fiance has a Blackberry. He doesn't play any games, uses the web maybe a few times a week to check sports scores. And he needs to charge his phone every day regardless.