This is a debatable topic. Personally I don't use any Task Killers. The official stand from Google is Task Killers are unnecessary for Androids.
I quote from Google :

By default, every application runs in its own Linux process. Android starts the process when any of the application's code needs to be executed, and shuts down the process when it's no longer needed and system resources are required by other applications.
A content provider is active only while it's responding to a request from a ContentResolver. And a broadcast receiver is active only while it's responding to a broadcast message. So there's no need to explicitly shut down these components.
Activities, on the other hand, provide the user interface. They're in a long-running conversation with the user and may remain active, even when idle, as long as the conversation continues. Similarly, services may also remain running for a long time. So Android has methods to shut down activities and services in an orderly way.

Refer here for more information:
http://forum.xda-developers.com/showthread.php?p=6426198
http://geekfor.me/faq/you-shouldnt-be-usin...r-with-android/
Watch this video and brief summary of it :



Almost everyone has used these at one point in their Android life. And in most cases, reps at carrier retail stores will install these upon buying any of their Android phones with or without your knowledge. Its part of this HUGE misconception that Android needs these to run properly. Couldn't be further from the truth.

Everyone knows one of the main benefits with Android is the fact that it, unlike certain other OS's, can run apps in the background. And the more RAM on your phone, the more apps you can have open at one time for true multitasking. This is one of the reasons RAM is a highly sought after spec when comparing Android devices.

Most people know Android sucks when it comes to battery life. It's definitely a problem in Android. We know this. The misconception a lot of people have is that the reason for the horrible battery life is largely in part because of all the apps "running" in the background. This is one of the reasons these misinformed reps install these Task Killer apps onto phones thinking they're helping, but they're not. First, Android doesn't necessarily keep apps "running" in the background like a car. Using gas/CPU without you knowing. Android will pause those apps until you return to them. If you don't come back to the app for awhile, Android will eventually close the app altogether.

The other misconception people have with using a task killer is that it's some how making their phone faster. This usually has something to do with the thought that "freeing up RAM" is going to make your phone speedier and quicker when doing everyday tasks (I used to believe this as well). But not so. First off, even if you close out 20 apps in the background, 20 more will open up in their place. Like Bebe's Kids. They don't die -- they multiply. Resistance is futile. Second, it's not like Android keeps running apps in the background until you're completely out of RAM. That would cause your phone to completely freeze and lock up. Instead, it will begin closing the oldest apps still open in the background to make space for new ones. I mean think about it, do you have to manage the RAM on your home computer? No. It just works, right? Same with Android.

But just like your computer, there are those times when things go wrong. Sometimes an app can misbehave and become unresponsive. Or perhaps you changed some options in an app and it needs to be closed/killed/stopped so that it can be opened and refreshed. Or maybe you have a game that has gone rogue and really is, in fact, running in the background sucking CPU and RAM and causing your phone to run to a crawl. It's only in these rare occasions that using a task manager is acceptable. Even then, for your average user, a simple reboot will solve these rare problems.

So for techy people that like to monitor and micro manage everything that goes on in their phone, yes, using a task manager is acceptable. But for your average soccer mom/gf/grandma/Joe Schmo, there really is no excuse for them to have these apps installed on their phone. More often than not, they're going to ruin their user experience with the phone, doing more harm than good, and it will cause them to run to the iPhone where they don't have to worry about this sorta stuff. So with that. I am done here, folks. I hope this Public Service Announcement helped shed some light on a murky subject with Android. Now go uninstall that Task Killer. Now.