I had intended to make some progress on zsync over the holiday period, but, perhaps wisely, I decided to give it a break instead :-). So there has been little progress since december. However, I have been working on a few bugs that have cropped up.
zsync 0.1.6 is now available from the download page. Most importantly, it fixes a corner case in the compressed file download which caused compressed file transfers with large differences to miss blocks and potentially fail, or at least transfer data less efficiently. I have also fixed a problem with servers that return fewer ranges than requested, and zsync not parsing common http_proxy settings.
And, yes, zsync is now in Debian. I am still interested in positive feedback by the way, particularly if you have any good uses for zsync.