Useless.
Android managers it's memory in a different manner than we're used to. While android does keep apps in memory, it is only as a caching intent. If the app is commonly used it is kept in memory to improve app performance. If the memory becomes needed by another app, the cache will be dumped.
Task killers often advertise that they save battery as well, but it's 99% incorrect. While an app is stored in android memory and not currently using CPU cycles, it does not use any additional power.
The contenders for best way to get better battery life is usually either reduce screen brightness, switch to wifi when available, or remove carrier installed "bloat" apps. (In no particular order)