I use ZNC now for a while and it’s really nice. It works and it does what it has to do.
I used ZNC with different clients and it worked very well. One thing was the problem to auth a user to ZNC. You have to put your username and the password into the clients "password" field ([Username]:[password]). I thing this is for experienced IRC BNC users something normal but not for common people without experience in doing such things (and sometimes with really strange client programs).
From my point of view as an administrator of a Server with ZNC, I miss log data. ZNC doesn’t produce any log files. This is a huge problem for me because I never know which part of the connection was the problem if some users aren’t any more connected. It would be useful to have some data like "Server XY closed the connection." or "[some ZNC alive information] […] ZNC started." With such information I would know what happened and could try to fix it.
[Edit: I added a feature request for the mentioned problem. So if you like to make this software piece nearly perfect: https://sourceforge.net/tracker/?func=detail&atid=672824&aid=2387499&group_id=115828]