I and a friend of mine stayed at Sonja's apartment for a little more than a week. The location of Sonja's place is perfect: near to all sights, shopping--tip for groceries: food market (Is-Suq tal-Belt on Merchants St), and the bus station. The apartment is beautiful, with all basic appliances (including hairdryer, stove-top espresso maker, and a small washing machine). Communication with Sonja and Trudy (the caretaker of the apartment) was perfect over email and whatsapp--thank you both for your flexibility.
A genreal remark on the climate on Malta: it is extremely humid and hot in the summer. Therefore, the closet in the upstairs bedroom smelled a little of mildew, but I don't think there is anything that could have be done. I preferred to just leave my clothes in my suitcase and that was perfectly fine.
I definitely recommend Sonja's apartment.