How much battery savings to expect from SetCPU?

Has anybody done any serious measurements as to effects of SetCPU (or a similar power-saving app)? I mean, are Google and Linux developers really that stupid not to properly implement power management in-system? Especially so considering the Android's paramount problem of battery life. The SetCPU promo video only shows how the MHz numbers jump up and down, but is it noticeably better for the battery than processor idling done by the OS itself?