I loved staying here. The apartment is immaculate, comfortable and in the perfect spot to explore not only Valletta, but the island itself. Almost all the bus lines are accessible from the nearby bus terminal.
The apartment has air conditioning which is a must when you visit in the summer months.
One thing to note, be sure to bring bug spray as Maltese mosquitos are quite sneaky and get inside even when the windows are shut.
But apart from that, I can't recommend Frank's place highly enough.