I had the phone rebooting and freezing issue a few times. I had to restore the phone back to factory settings within the first week. I have since removed advanced task manager and have not had any issues with the phone rebooting since then. I was told the app killers can mess up the phones normal processes and cause issues by a tech with verizon. Good luck. I love my DroidX too so far!
I have yet to install a Task Killer and I have yet to feel like I need one. My phone has never slowed down or frozen. I do periodically check (like once or twice a day) the open applications manually to see if someone is running that I don't want and one time Pandora was still running even though I wasn't using it anymore - so I closed it. Even then, it wasn't slow or anything.
My co-worker with a Droid Incredible uses ATK and constantly complains about it freezing or slowing down. I told her I didn't install it and that a lot of people online say they are unnecessary and she told me the Verizon rep installed it on her phone in the store and told her she needed it.
Shame. She was going to uninstall ATK and see how it does. I have yet to see her this morning to see if there has been any change, but I'll post back once I see her.