It's quite a cool line. It's the only metro connection to Barcelona Airport, although if you're going to the center, it's a bit of a detour, as there are more direct routes.

Since the line is completely automated, you can sit at the front or back and get an unobstructed view of the tracks/tunnel.

If you're into trains/trams (like a certain founder of Pied Piper), it's worth checking out.