Flat view unless I want to search for something in a multi page thread. In that case, its much easier to use the nested view.
Example: If you are in the "Today's Smoke December" thread and want to see who smoked a Perdomo, the flat view brings you right to the last page and a search (CTRL+F) will only uncover 2 Perdomo smokers, but change the view to nested and you will get 22 (a search of the entire thread).
Just a note, as I'm playing around with nested view.
Replying to a post vs. replying at the bottom of the page makes a difference. I think a lot of people hit reply on a post, when they are really replying to the thread, so it kind of messes up the parent/child functionality...