A few months ago, I wrote on Twitter that I never edit a manuscript while I'm writing it. I get everything out and then when I'm finished, I edit it to death. Why?
Well, I find editing while you're writing to be very distracting. If you keep changing something in Chapter 1, how long will it take you to get to Chapter 3? I like to write three to four chapters a day, so editing while I write isn't good for me. One thing I do recommend is keeping a list of things you may want to change later, but only add the changes when you're finished. It'll give you a chance to look at the work as a whole, not disparate parts.
What works for you?