I've used vim + emacs's undo tree quite a bit. It's really nice to be able to make a bunch of explorative changes to the code base, undo them, make another set

I've used vim + emacs's undo tree quite a bit. It's really nice to be able to make a bunch of explorative changes to the code base, undo them, make another set of changes and then use the undo tree to recover interesting bits from another branch.