In the last half hour I've installed an update to "battery left." My Droid immediately jumped from 49% left to 52% left. Then after installing five more updates it dropped immediately from 52% to 40%. Then after hooking it to the wall outlet charger, it jumped almost immediately from 40% to 51%.
In short, battery monitoring software is highly variable and shouldn't be taken too seriously.
() It tends to vary according to a particular battery. I have two and they differ significantly in their "draining profiles" though they last about equally.
() Battery use is not linear. The amount of "drain" from 100% to 90% can be vastly different than the drain from 60% to 50%, etc.
Battery use is HIGHLY dependent on the phone's use (not surprising) and not always on the use of apps you'd think would be using the battery.
() Installing/updating apps is a battery killer. I downloaded 35 apps for my wife's droid and completely drained her battery in two hours. Since then, she routinely gets more than 30 hours out of her battery.
() Synching can be a battery killer. If you check your email, facebook, and messages every 15 minutes you can kill your battery, especially if you are either (a) in a fringe area trying to use 3G or (b) have a weak wifi signal while using wifi.
() Updates to news and weather can kill a battery. I generally set my news apps never to update. When I want to find out what happened in the world, I open the app and get the latest news.
() As already noted, use of the GPS for any sort of navigation should be done only if you have an extra battery on hand. There is a reason that almost all GPS units are larger than the Droid. They have bigger batteries.
() Keep your screen turned off when you're not using the phone. Screebl does a great job substituting for an obsession with turning it off.
In general, investing in a second battery for less than $10 and a standalone battery charger for about $15 from amazon.com can eliminate most of these concerns. I get between 25 and 30 hours out of my battery and simply switch batteries every morning.