When I got my phone I installed task killer and it was always speedy and responsive, the I uninstalled it as people say it didn't need one, eats up the battery all the usual blah. After I removed it I found my phone would become so slow and sluggish and I really didn't like what I was seeing. So I put another task killer back in and everything is fine again.
Battery lasts for ages, always have a speedy interface and anything that isn't vital or not being used gets shut down so when I next go to do something it's ready to go.
People will argue both ways, for me having tried with and without a task killer many times, I am going to be sticking with mine.
If anyone is interested I use ES Task Manager (as well as their file manager) and they are both great pieces of software.
To anyone that does use one you must make sure you know what you are doing when it comes to adding items to the ignore list, otherwise you may end up with no advantage at all.