On my netbook running Ubuntu 11.04, Jupiter is quite useful and allows my computer to run in Eco mode when disconnected from AC power. Without it, I was getting a little over 3 hours from the battery. With Jupiter installed and running, I've gotten up to 4 hours... 5 when running the proprietary ATI graphics driver (which sucks ar running Adobe Flash).
I'll vouch for Jupiter. It's nice that Fuduntu comes with Jupiter preinstalled