Milestone actually didn't support my phone after rooted. Tried using the free version of CPU Master and I can only make it stay at 1000 instead of scaling.
I've heard a lot of bad things about overclocking phones. Many of my friends are having issues with overall stability when overclocked. I will probably be a little reserved on overclocking when/if I get an Android phone.
I only have mine set to OC when I have it plugged in (which when I'm home, is most of the time). I've set it to downclock when the screen's off (only down to 800MHz) and down to 400MHz when battery is <10%.