I have run ATM and TasKiller (pro). Of the two I prefer TasKiller Pro.
I have run the system with it for 3 days and without it for 3 days. I am a heavy data user and have about 8 email accts syncing to it. I use messaging for both sms & mms. I use Pandora all the time. I make phone calls. I stay on the phone an average of about 2-3 hours a day due to work and some personal calls.
I take pics with it and send it. Even made a 83+ meg video of my son's case being cut off.
I cant tell a difference in battery life by keeping apps killed and just letting them run.
This is a linux based OS and I have been running linux off and on as a hobby OS since mid 90's I am not a linux guru but I do know that I dont have to manage the apps that run on it like I do windows.
I do know that Android handles its resources far better than WinMo, Symbian OS, and BB (Java is just bloated anyways).
I will keep TasKiller installed but not running just to kill an app with one to clicks instead of many by going into settings and such.
Everybody will believe what they will and want and will not listen to others unless they want to. But as an avid tech geek and being in the IT world for some time and having a logical overview of technology.
I say trying to keep every little single app killed on the Android is a bit silly. You will make yourself believe that it is helping you. Even if you have all 256 megs of storage used for apps and what not I doubt you will see a huge improvement on performance.
But to each is to their own on this and enjoy your device how you want to.