As a long time CVS user, I find that most of my .cvsignore entries are there to keep CVS quiet about build products and various intermediate files that come about from testing (gcov data, and so forth). It makes it easier to tell at a glance that all the important files are accounted for.

Given the fossil extra and fossil clean features, I've already felt the lack of some way to inform fossil that I know some files exist but that they are not "extra" and should not be removed by "clean". Without that, I'm really tempted to patch the fossil clean out of my copy.