Multiplied db_results when multiple IN
Hum... Classic "I am sure I tried that before and it wasn't working"... I spent an entire day looking for the solution, and find it 5 minutes after posting here :)

$query = db_query("
  SELECT n.title, SUM(q.quantity_value)
  FROM {node} n
  JOIN {quantity} q
  ON q.entity_id = n.nid
  WHERE EXISTS (SELECT *
    FROM {tags} t
    WHERE t.entity_id = n.nid
    AND t.tid IN (12, 15))
    HAVING COUNT(DISTINCT t.tid) = count(12,15)
  GROUP BY n.title
");
$results = $query-&gt;fetchAll();


Subquery was the way to go (if anyone has something better to suggest, I would love to hear it).

Thanks,

Ed