This is the perfect, no-hassle, venue for both Sliema and Valletta itself. The apartment is exactly as described and Alfonso and his partner make the check-in totally painless. It isn't the height of luxury, but who cares? How long will you actually be indoors when, once onto the street, real Malta is laid out for you.
At the bottom of the road, literally, is the Sliema to Valletta ferry, which costs almost nothing to deliver you speedily to the heart of the town (even cheaper as they are happy to give 'age' discount - assuming you qualify)
A colleague stayed at the Waterfront Hotel, just down the road. I visited once... you might as well have been in Coventry, just a standard hotel from anywhere in the world, full of Brits. Real Malta is here, up the side roads.
Downsides? Almost none. Maybe by the time you get there Alfonso will have found a plug for the bathroom washbasin ;)