Looks great! Couple of first impressions ...
1) Had to apt-get remove --purge an older version and reinstall xulrunner to get it to run on Ubuntu Natty. Just updating the package from the PPA led to a non-working binary. YMMV.
2) Search - as we discussed in Berlin it would be great if we could filter out redundant titles from the index.
3) Downloader - looks fantastic already! Really nice to see this out there.
4) Potential major BUG: I downloaded a file (Urdu WP), started indexing, then downloaded another (Swahili WP) and loaded it. It prompted me that an indexing process was already running. When the index process finished, however, it seemed to associate the index of the Urdu Zim file with the Swahili one. So, now I can search on the Swahili file, and get Urdu results. :p
5) Possible bug: I had to manually associate external links in Kiwix with a browser. May have been GNOME funkiness on my end.
6) UX: I would suggest adding the Library to the File pull-down menu, so it's not just accessible via the button.
Again, really coming together nicely and the library is a real killer feature :-)
Erik