In my project I'm using it to connect to a database that heavily uses some of the advanced PostgreSQL functionality, including PL/pgSQL functions taking arrays of records as arguments. I found S-SQL to be a little lacking in expressiveness for that, so I've quickly hacked some methods that let me pass Lisp vectors to SQL and QUERY (they will be rendered into PostgreSQL array literals):
on a somewhat related note, i'm working on extending/reworking the binding mechanism so that users can add methods for lisp types that serialize lisp values directly into the socket stream.
for that i've added a serialize-for-postgres generic (that by default calls to-sql-string), but i need to spend more time to think through how this could be integrated better (e.g. i'd like to support serializers without an intermediate byte vector buffer). but i'm not happy with the current code yet... although inserting local-time:timestamp's without turning them into strings works already.