My main reason for rooting and applying new ROMs was to try and improve battery life, because I was getting annoyed that my battery could die inbetween me getting up and going to bed even with just a moderate amount of GPS use in the daytime. I tried CM7 to start with, but suffered force closes quite a bit and thought I should try some other ROMs anyway. Then I tried RCMix Energized, which looked good, but didn't seem to help the battery situation. I'm now on Oxygen and although it's very basic, it still has all the features I need and there also appears to have been an improvement in battery life, but I've only had it on for a few days. So far though it's looking hopeful.
It's dead easy to try a new ROM as long as you backup your old one before you do so, as you can just reflash the backup and get back to exactly where you were before.