DVCS - How often and when to commit changes

Question

There is another thread here on StackOverflow, dealing wih how often to commit changes to source control. I want to put that in the context of using a DVCS like git or mercu

Answer 1

I follow this kind of flow
alt text http://img121.imageshack.us/img121/3272/versioncontrolsysbestpr.png

(Here is the original image url)

I guess this says pretty everything. Basically I would do a check-in after implementing a full working use case / user story (depends on your software process). The major important thing is that you check-in things that work in the sense that they compile. Never break the build!
Doing a commit after each user story/use case has the advantage that you have a better tracking of past versions and undoing changes is much easier.