This is a very touchy subject in other forums. Here is Androids take on app or task killers.
YOU DON'T NEED THEM!!!
Here's why (you didn't think I wouldn't tell you why did you?), when Linux designs an operating system built in is memory allocation. The running apps or tasks are running for a reason. When another running app or task needs more memory the OS will move more memory to that app. Android is of course built on linux and runs just like this, therefor you don't need a task killer. When an app has run it's usefulness in this instance it will automatically shut down.