My original iPhone and iPhone 3g were both jailbroken prior to getting my iPhone 4. Used the 4 extensively prior to jailbreakme. It had the most amazing battery life. I literally was blown away.
Then I jailbroke it. I kept it simple. I really do it to tether, use sbsettings, quickreplysms, and to maximize audio and adjust vibration patterns.
My battery has blown since the JB. I saw 10% drops over a six hour period without use. That's after killing processes that were running in the background.
I decided to remove some things. Same problems. Eventually, I just removed MobileSubstrate. This also removed most of the customizations I had (sbsettings, quickreply, and 5 icon dock).
BAM...battery life back to normal.
Yes, I miss those three things. But having an amazing battery beats it. The only thing I wish I could get is the Saurik PDF Fix....but, alas...it uses MobileSubstrate.