If you root the Evo then install a custom ROM + unlocked kernel + underclock the CPU when the screen is off you can probably get a day of medium use. Do note that the Evo likes to show fully charged when it's not. It'll charge to 100%, say it's fully charged, but really is running off the battery till it gets to 90% to start the cycle again. For example, this morning when I unplugged my Evo, it dropped to about 90% within two minutes. I plugged it back in while I got ready, and was in the car with it at 94%. 15 minute drive with bluetooth OBDII monitoring, screen on, and 3G streaming audio (almost worst case scenario) I had 89% when I got to work.
If I just do audio streaming to and from work with a 100% battery, leave the phone in airplane mode in the car while at work (9 hours), do not talk/text/email on the phone, underclock to 325 MHz while the screen is off, bluetooth always off, and leave the screen at 20% brightness and have it off nearly all the time I"ll usually be at ~70% battery after a total of 17 hours off the charger. Of course, that's one step away from not using the phone at all
So, yea, if you're not planning on rooting the phone and customizing it you won't be happy with the battery life. With some tweaks, though, you can get it to last all day with medium usage. Keep in mind the 3G/4G radios are the big killers in the Evo.
Oh, and you won't be able to force it to roam on Verizon without using a hacked PRL or buy an app for I think $4.99 or something. You need root in both cases, and only the hacked PRL will get you 3G access on Verizon. Whenever I tried the app or let the phone automatically roam, it wouldn't connect to Verizon's 3G side of the network.