Actually, I have a Sprint HTC Evo and the battery life really IS a problem.
I can BARELY get through a work day with mine, starting out with a full charge when I take it off the charger right before leaving for work. If I get a large number of phone calls on it, or a longer call than normal? It's usually done for before I get time to get home and plug it back in again. I have to use my car charger to keep trying to "top it off" if I go out to lunch during the day, or on the commute home if I expect to be able to take any calls with it in the evening.
Here in St. Louis, MO, I don't really see much benefit to leaving the 4G mode on? When I do, it usually just says it can't find a 4G signal, or else it does but my speeds seem pretty similar to what I get with 3G, and then it suddenly drops out on me. So I agree that 4G usage isn't what's using the battery up.
But I could leave bluetooth and wi-fi and GPS services on all the time on my iPhone 3G and it lasted longer on a charge than this EVO does. Sure, it'd be "dead by the end of the day" too -- but that would be after I used it pretty heavily AND my kid played games on it after work, when I went to pick her up from my mom's place before actually driving home. I didn't ever feel a need to buy a car charger for it, and I never felt nervous about "talking on it too long" during the middle of the day and risking it dying before I was ready to plug it back in someplace.