Sergio's flat is superbly located for everything you could want in Lisbon - near to amenities and sightseeing spots. The balcony is lovely for getting some sun during the day, and there is every possible thing you could need, such as a kitchen and TV.
Sergio is an excellent host, and has a "secret weapon" - his great guide to Lisbon which means you don't need to do any planning beforehand. Just follow it, district by district, and you can easily fill more than a week in town before you know it.
Highly recommended!