It's failed a few times recently on sites where I had entered a lot of stuff into a text field.
It should be a useful feature where the post/submit button often fails, and the site is poorly designed, so when it fails, the drafted post/comment/etc to be lost. But IT fails! Silently.
I just tried it here, on this page - posting.php?mode=post&f=14
and it SILENTLY FAILED! It saved the subject and a few other fields. But this one - the one that took the most effort to fill? NOT SAVED!
(I tried it here
, for test purposes only - this page has a Save draft button...)