I was interested in this thread and decided to try tracking my own battery progress starting yesterday. I charged the phone to 100% on 11/14/2009 and began keeping track of battery level periodically after that.
Active apps included Mail, standard message service, Flexilis mobile security, Gmail, Facebook, Audio Manager, Battery level widget, Missed Call, Weather widget, Wi-Fi turned on, GPS Off, Blue Tooth Off, and Advanced Task Killer installed
15:11 100% removed from charger
19:15 92% 10 min of calls, about 10 texts, installed battery graph
12:33 69% periodic internet use and using Facebook
03:30 51% periodic Internet use, a few texts and browsing wallpapers
10:30 36% just woke up and checked phone
11:00 checked some phone info: Up Time: 135:50, Awake time: 22%
battery Level 4 bars, from secret menu: 35, Health: Good
12:30 28% occasional fooling with phone and settings
12:45 24% installed Handcent Messaging and messed with settings
16:25 14% low battery warning alert with flashing LED
19:15 03% after 1 call, 8 texts, 10 min showing Eris features to a friend
I also added a free version of a Power Manger app with preset profiles before going to bed at 03:30. Once the phone reached 30% it turned the WiFi off, the screen dimmed quickly and the phone turned off quickly when not in use.
Considering some of the other times and levels posted here, I am quite pleased with the overall performance of the battery. I had quite a few apps running and the advanced task killer installed, and my battery lasted much longer than some of the previous listed users who had most of there apps turned off. Being a new phone and the fact that I still havent figured out how to use everything, I am constantly turning the Eris on and messing with it.
As noted I ended up getting approxiamtely 28 hours from a single battery charge. I am perfectly satisifed with the battery performance considering how thin the phone is and how much I am messing with it.