FDS is +1 for me (Score 1)

As far as my experience tells, Fedora Directory Server has my vote. Very mature, good integration with Windows and Unix world and (very important) great admin tools and interface. I've been using it for 3 years, and never feel the need to go do RDS (the supported version). openLDAP ispretty rought, even if it works well. No idea on Apache DS. If you want to have the list of all candidate solution, have a look at http://www.opensource-it.com/tags/directory_server_0