Verdict is to purchase the battery for $5 to $14 dollars and use it. It's worth the money. But let me tell you that if you use Navigation a lot with it don't expect much life out of it or any battery for that matter. It does eat them up.
Outside of that I get a full days worth and then some with the battery so far. As I have mentioned before I get a lot of news feeds. BBC, Buzzbox, Friendstream updates, Active Sync, Gmail 15min polls, News and Weather updates, Drudge report,NYT, and a few more. Some of the them are active pulls/push.
The battery indicator(stock) might show you that you are yellow but its still over 30% left on the battery. It's not very accurate. Use a market battery meter to see the percentage.
I purchase a couple. I will be using one on my EVO 4G as well. If the Sprint service works out for me then and I like the EVO 4G enough I will move to one phone. I have been carrying two phones around and its just a pain to check on both all the time.
Don't hesitate on the battery.