SunOne Directory Server is very stable, more than OpenLDAP, but may be not too fast. (Am I wrong?)
http://www.daasi.de/staff/norbert/thesis/
Critical Path directoty server is allright, stable, fast and everything Netscape Directory, well the same SunOne... Active Directory from M$oft, ahmmm no comments.
Hmm, I doubt that you have much practical experience with all these products...
Ciao, Michael.