I've been looking at a number of scripts, some that integrate with WordPress.
It's interesting how they work, using WP's registration process to manage user additions to their directories.
I've also been looking at scripts that are NOT integrated into WP BUT that offer . . or suggest they can be integrated.
Among the WP integrated scripts some say their ability to scale is ~unlimited. I somewhat believe the assertion because scalability is more a function of MySQL and DB optimization than it's a function of the script (assuming that it's well coded.)
Anyone care to share their experience with directory scripts that are brought to market as WP plugins?