First off, here's what my usual (average) usage is on my Droid:
2 email accounts - Gmail (push) and another personal account which syncs every 30 minutes. Pandora running between 1 and 2 hours a day. Web browsing for another hour or so, this fluctuates. Sending/Receiving txt msgs (varies by the day)... I have Google Talk open and chatting with my wife and a few other friends throughout the day. PhoneMyPc usage of about 20-30 minutes daily. And about an hour of talk time per day. Not to mention I also have Locale running at all times to set my sound and notification preferences throughout the day.
Wifi is OFF. Bluetooth is OFF. My Cellular Signal while at work for 9-11 hours a day is around -95 dBm. The rest of the time if I am at home it is usually -64 dBm (tower is visible from my house).
Now, I have 2 batteries. One that came with the phone, and one I ordered from VZW a day after I purchased the droid. I also have the single battery charger which I use to charge my batteries. I put a freshly charged battery in my phone daily at 6am. I then take the battery that was in the phone, and leave it sit all day. Right before I go to bed, I put the battery on the charger so that it is ready in the morning.
The battery that came with the Droid will be at around 50% at 11pm, and usually around 40% when I wake up in the morning. The 2nd battery will be at 80% at 11pm and maybe 70% the next morning. Since today was not a work day, I have left the 2nd battery in since yesterday at 6am and just now it is sitting at 30%.
I think there may be some batteries that are not quite up to par and this is to be expected. My original battery has been getting better, as I have only had the phone for 2 weeks now. However it just can't touch the 2nd battery. For a smartphone, this is still not bad. The WinMo phones I've had in the past were usually lucky to make it 10 to 12 hours before it was completely drained. However the blackberries I've had usually lasted anywhere from 3 days to 7 days (yes, 7 days of use!). However looking at what the Droid can and is doing, getting almost 2 days out of a battery is just fine with me.
I recommend those suffering with less than 16 hour battery life should see if they can get the batteries replaced. Unless you are a super user and on the phone constantly from morning til night, you should be able to make it a whole day without needing to charge. Again, everyone's use will vary thus will battery life.
Happy Droiding!