Both hosts, Mark and Sybiel, were more than willing to help us at any moment. They even texted us when we were taking the plane to Malta to wish us good journey. At some point we needed some blankets, so we texted them and within an hour they brought six of them (more than enough for the four of us). You could not wish for better hosts.
The apartment is not very big but it definitely matches the photos and the description, and is located in a wide street in which it is very easy to park. Besides, it was vey clean and beautifully decorated.
Marsaxlokk, in my opinion, doesn't have much to see, but it's just 12 minutes away from the airport and, given Malta's small size, very near to from most touristic spots. It takes just 45 minutes to get to the other end of the island. Also, take into account apartments and hotels on Valleta os Sliema of the same quality as these one tend to be more expensive.
All in all, I definitely recommend staying with Mark and Sybiel.