Our stay in Filipes Appartment was just great! It is locatated in the heart of the center - not just close to it! It might not be the biggest but you have everything you need! For this price you can't find something better! I'd definitely book again next time I am in Lisboa!
Besides that Filipe is a really nice host and answered all my questions immediately! He also made his own tourist map for a lot of portuguese insider info which you can find in the Appartment.