Reliability of CURRVAL in a RULE

Is the use of CURRVAL in this example reliable in heavy use?

CREATE TABLE users ( id SERIAL NOT NULL, email VARCHAR(24) DEFAULT NULL, PRIMARY KEY (id)
);
CREATE TABLE users_with_email ( id INTEGER NOT NULL
);
CREATE RULE add_email AS ON INSERT TO users WHERE (NEW.email IS NULL)
DO INSERT INTO users_with_email (id) VALUES (CURRVAL('users_id_seq'));

I tried...

CREATE RULE add_email AS ON INSERT TO users WHERE (NEW.email IS NULL)
DO INSERT INTO users_with_email (id) VALUES (NEW.id);

which was incrementing the sequence twice. Should I be using a trigger
instead? This rule seems quite simple and easy enough... if reliable. -
Nick