My own experience is if I have the screen on (i.e. using my Mesmerize constantly), I go through ~10% per hour. When not I use, I guess it is around 2% per hour. If I'm using apps that use GPS, bluetooth, wifi, and so on, my usage might be more.
When I'm at my work desk, I have it on a cradle charging. It is in a cradle in the car as well, so it's plugged in there, too (mainly so I can use GPS). Between that and charging overnight, I haven't run out of juice. I came close when I first got it, but that was because it rarely ever left my hand.
It was just about the same with my BlackBerry. I used it so much each day that it wouldn't make it through an entire day (carried a spare battery with me for my Tour). I got into the habit of putting my BB in a charging cradle on my desk, and had a car cradle that charged as well. It was a natural transition to do the same thing with the Mesmerize.