Yair Friedman <yair@cs.huji.ac.il> writes:
   Does anyone has experience with setting nntp-nov-is-evil to t? What
   exactly will I gain or lose by setting this variable?
   
   Setting this variable only makes sense if netnews is set up on your
   system to not provide XREF (cross reference information) in the
   overview files Gnus reads to build group summaries.
   
   A quick test to see if you need to use it is this (substitute your
   nntp server, and use the last number returned on the 211 line as the
   parameter to XOVER):
   (Example courtesy of Christopher Davis <ckd@loiosh.kei.com>).
$ telnet news.somewhere.com nntp
LIST overview.fmt

You should see something like the following (on INN systems anyway):

215 Order of fields in overview database.
Subject:
From:
Date:
Message-ID:
References:
Bytes:
Lines:
Xref:full

   If the Xref:full is missing, ask your news administrator to add it in.
   
   Assuming you have a broken newsfeed, you gain by setting
   nntp-nov-is-evil the ability for Gnus to mark all crossposted articles
   as read the first time you see them. Otherwise, every crossposted
   article is spam, and you see it over and over again if you also read
   the other groups it is crossposted to.
   
   You lose the ability to have Gnus fill in old headers to flesh
   partially read threads with old articles. This may not matter a whole
   lot if expiration times are short. Building group summaries is also
   somewhat slower, though if your network connection to the news server
   is decent this isn't a huge loss.