I'm sorry but I can't keep my mouth shut anymore. All this talk about not receiving emails with a task killer is wrong. I use an app called "Advanced Task Manager". All you have to do to receive your mail is go to the Applications tab and long click on the app that you do not want to close (for example: Gmail, email, etc) and select EXCLUDE.
You can also auto close applications at a specific interval, 30 min, 60 min, etc. To do this go to menu/preferences/ and check Auto-end running apps. Also make sure you go to auto-end excluded list and ADD the programs you do not want to end such as email programs, maps, open home(or equivalent), etc.
If you take the time to set this app up properly it works great, and IS a battery saver. If you don't believe me then try it.
For other battery savers, go into each individual mail app, weather app, etc and set the updates to a longer period, rather than updating every hour or 30 minutes.
Also set backlight to a low setting, or use auto adjust, and select a shorter backlight timeout value, like 30 seconds.
I have done all of these and my battery life is awesome. I love the Droid, my fav phone yet.
Sorry to be rude but using a task killer or any other app is left to the discretion of the user. No one should be told what to use or what not to use. Try them out and see for yourself if it is for you or not.