It was "inaugurated" on 30 November 2010, and I have seen a photo taken June 2011 which showed about 6 sailboats moored up to a few of the vast number of places available.
Yes, it is fully open, and open to visitors. It's planned size is around 700 berths but is now mostly empty (maybe max 100 berths occupied??) so there is no shortage of space for visitors.
I live in the Basque Country so am reasonably up to date on developments in this area. Laredo is just to the west of us, and a useful stop on the way to Santander or further west, and also for a weekend trip. The old arrangements there left you sitting on a mooring buoy in a fierce tidal stream on the edge of the main channel to Colindres, the fishing port, and on the wrong end of the Laredo beach, with the town and facilities, night-life, etc at the other end, maybe 3 km away. The new marina is right in Laredo town itself.