Just set it and forget it
It also tracks the saved cpu consumption, but this is a mere estimation. Another really neat feature is the "Game Mode" where all applications besides the top front window gets paused giving you the max possible FPS in Starcraft II.
Paused applications get grayed out so you actually see when its paused, the suspending and reactivating happens without any noticeable lag and so far App Tamer worked nice with all my custom system add ons like Total Finder or Hyper Dock.
While on a desktop the advantage is nice but not always necessary on a laptop the application is a must have, since recovering even a small percentage of your CPU time can translate into significant battery saving. With App Tamer my batteries last around 25% longer which means more then 1 hour a day till i need to plug my MacBook in to recharge.
The interface is well designed and easy to handle and it runs rock stable even on Lion, big + for this.
I'am using this app now for 1 week non stop and i just love it.
You can try it free for 15 days, afterwards a single user license costs $14,95.