Straw poll everyone!

I'm curious. Are you a person who goes back and tweaks what you've just written right away? Or do you wait until you've got a full first draft behind you before diving back in? Are you somewhere in between?

For my current writing project I'm waiting until I've got that first draft. From bitter experience I know if I go back to chapter one for a quick edit I will never get out. It's like a black hole that sucks me into its orbit and before I know it I've re-written the first three pages from scratch.

So where do you land? Has anyone drifted from one way to the other over time?