I had an EVO and I have a couple of batteries I use from time to time they work fine and last me just as long as the stock batteries. If you are having issues it might be because the batteries are old.
1. one of the batteries that shipped have gone bad or have less life
2. Mango needs calibrating
I've pulled it from the charger, and so far it is outlasting my work iPhone 4. My phone reads 100% (with 1 day and 18 hours left), while the iPhone is 99%. Both have WiFi on (with the auto sleep feature that activates to turn off WiFi when the devices are sleep that can't be turned off....grrr) and I've used my phone a tad bit more (for texting and such) and have my Gmail set to "As Items Arrive", whereas the work iPhone 4 only has Exchange on push (which my Arrive has Exchange on "As Items Arrive" also).
So....either it REALLY needs calibrating, or the other battery I had in yesterday was bad....or a mixture/combination of both.
Yes, the EVO battery is interchangeable with the Arrive. Also the Touch Pro 2, Hero, and 1 or 2 others will work in the Arrive. I even have a 1900mAh slim battery from htcexpress.com for the EVO in my wife's arrive. It gives her a few extra hours of battery.