Hi,
I know people who use both. Clearly if you're a Windows shop there is going to be a lot tighter integration between out-of-the-box tools that run on that platform and Active Directory. Also hard to come up with a good reason not to go the Active Directory route if you're already managing all of the organization or site's users in a Windows environment, since you're really not going to be able to effectively replace all of the functionality of Active Directory in a Windows environment without being very invasive by changing around DLLs and such.
On the other hand, going to a pure LDAP will allow you to take better advantage of some software that comes out of the box with LDAP support (policy servers,
Java application servers, etc...). If you're developing primarily Java-based applications to use the directory, pure LDAP is almost always the best way to go, since certain functionality in Active Directory is not available via LDAP and therefore can not be accessed via pure-Java APIs such as JNDI and JLDAP (only ADSI, so prepare to use Visual Studio .NET to write some JNI if your application or portal requires password self service functionality).
As far as server operation I don't see a lot of difference. Some non-AD LDAP directories certainly promise higher levels of scalability, particularly when used on high-end hardware that AD won't run. The old argument that AD is too tied to the OS is a little less persuasive these days because of Microsoft's application directory strategy.
Clayton
[ March 17, 2003: Message edited by: Clayton Donley ]