True story. Only thing you should be using a task killer for is to check usage to find any apps that really are hogging memory and get rid of them.
Every "best apps ever" site says you need one, so I installed one right from the beginning. My phone was very sad and it let me know this by freezing up constantly and running through the entire battery in an hour or two of using an app/killing it/using another/killing it. I came here and read all the posts that said Task Killers Are Bad, found out why, and went "Oh. That makes sense." I uninstalled ATK right away and started just using the settings/applications whenever I seemed to be having more problems than usual (though, after reading the above link, I'm trying out System Panel 'cause it seems a bit more convenient than my way). My phone is now mostly happy (it still gets grumpy when I try to make it work harder than an old phone should).