I use the Rotterdam metro quite often. It is easy to reach all hotspots in Rotterdam and moreover the metro runs all the way to The Hague and recently to Hoek van Holland harbor, almost to the beach! All from the center of Rotterdam! This is metro station Beurs (detail) in the middle of the city with one of the subway cars used for the Randstad Rail connections.
Travelling to The Hague? Take the E-line, direction Den Haag
Travelling to Hoek van Holland harbor? Take the B-line, direction Hoek van Holland