OK, my experience is based on the Sonicwall SSL VPN boxes (SRA line) which have different firmware. These have an option for Active Directory and LDAP (also local users, RADIUS and certificate auth).
(http://help.sonicwall.com/help/sw/eng/8112/8/0/0/content/Chapter14_Users.18.15.html)
Even in the case of a MS AD I ended up using LDAP as it was the only working way to get group matching and policies working this way. I also remember that even though they write (standard) LDAP that their Appliances still more an AD-like LDAP directory structure (attributes).