Well, this has kind of been beaten to death here already, but here's the short version of what I think.
The Android OS is basically designed to handle task management by itself, terminating tasks on its own as needed ... and I think it does a fairly good job of that. If your device is running slowly, the odds are that the real problem isn't the way that Android is handling task management -- it's just the fact that you have too many apps installed, or that you're running an app or two that's a memory hog. Take care of those problems, and you shouldn't need a task manager.
And remember that in most cases, the tasks that were left running were running for a reason -- so people who use a task manager aggressively run the risk of hindering performance by forcing tasks to repeatedly restart. Get too carried away, and you could also screw up things like notifications and stuff.
I know that some people claim that task managers improve their phones performance, but if they haven't overloaded their phone with memory-hog apps, I'm convinced that the vast majority of the time it's a placebo effect at work. People just like to feel that they're in control.
And that, as they say, is what I have to say about that.