It's worth digging in to the (copious !) documentation on
www.redhat.com/docs. I've only glanced through so far, but there is a section on Windows clients.
Note that LDAP is just one aspect of AD, so the products aren't directly comparable. It also provides Kerberos single-sign on for the accounts and systems it manages, as well as the Group Policy stuff for user/system configuration, RIS automated installation and app deployment. Red Hat have RHN and some great management technology in development for Linux clients that will use Directory Server, but you'll need to consider how to provide those functions for your Windows clients.
One technical advantage that AD boasts is true multi-master replication whilst Directory Server can only have four writable replicas of the directory (although unlimited read-only copies). Having said that, I don't consider AD robust and would have preferred it if MS hadn't tried to be clever at the expense of safety, especially when they couldn't be bothered to think properly about data integrity. AD is actually a mess of binary directory databases and policy files, so running something like Veritas is essential to recover it when one of those files is corrupted. Directory Server includes backup and restore tools, is proven to work efficiently with very large installations, and is less of a mess.
