I can understand why some applications are running, but then there's the Amazon MP3 app, and for the life of me I can't understand why it's necessary that it runs.
However, upon checking the details on these running processes/apps, most don't even use much memory. HTC music player shows 8kb, etc...the internet shows 10mb but after setting the cache limit lower that uses less now.
I'm not bothering with the task killers. It just feeds my OCD and I'll never get anything else done that way.
Not only do they not use much memory, which has no effect on battery life at all, they don't use much, if any, CPU when sitting in the background.
My take is that task killers "work," but they're the lazy person's way out, and they can be dangerous if you just set them to auto and let them go. If you let something like ATK kill apps indiscriminately, there's a real good chance it's going to kill important processes, causing some things to not work, and other things to be unstable.
Why the lazy person's answer? Because the real background battery hogs on the EVO are those tasks that use the radio to send or receive data. If you just accepted the default EVO settings, you've got news, and weather, and Facebook, and location services, and a bunch of other things running, and forcing the radio to eat up battery.
So you can use something like ATK to continually kill these apps every time they start up, or you can go into their settings and turn off updates on things you don't need (or extend the time intervals for things that don't really need frequent updates). If you look at the system panel app, the apps that aren't using the radio really don't use any resources when they're sitting in the background, so killing those won't make any difference in power consumption.
As an example, I started Maps and Sprint Nav, and have left them sitting in the background for 30+ minutes. Immediately after starting the apps, Maps showed 5s CPU usage, Sprint Nav 4s. Thirty minutes later, Maps showed...5s; Sprint Nav....5s. These apps, which people seem to think are important to kill, don't appear to draw any power when in the background.
For good measure, I turned on the GPS radio, and went into maps and Sprint Nav briefly to check my location and move around in the app a bit. CPU use in Maps went up to 12s, in Sprint Nav to 7s. About 25 minutes later, Maps is still at 12s, Sprint Nav at 8s. I can't find any entry for GPS separately. (For comparison, all the checking in System Panel has its CPU up to 2m7s.
The battery statistics don't show any independent usage stats for the GPS, either, btw. The heavy battery use in the battery monitor are "Android System", WiFi, Cell standby, Bluetooth and the Display.
When I first started reading about Android, I was pretty sure a task killer would be useful for increasing battery life. But real hands-on analysis of data just doesn't show me any advantage. There may be some badly written apps the continue to suck up CPU in the background, but the answer to that problem is to not use badly written apps. One post above complains about resource use by the Weather Channel app. If that's an issue, there are lots of other weather apps. I'm using Weather bug, and in the last hour+ I've been running System Panel to monitor applications, it's used a total of 1s of cpu. Oops, it's up to 2s. But that's still immaterial, compared to the real battery hogs.
If running ATK really improves battery life, you probably need to look at what it's killing, and see what those apps are doing.