We had a great time in the beatiful Aflama district.
The flat is small, but cozy and is directly within the heart of the district. Not very far to restaurants and tavernas.
The Metro, Bus and Tram stops are very close, so short trips to the sea or other districts are very easy. (But walking in Lisbon is so nice, you don't need those all the time anyway)
The room was clean and we had everything we needed, but if you are a bigger person you could encounter problems in the bathroom and the toilet itself is quite uncomfortable, because the lid does not open all the way and you can't sit up straight. That is the only, but annoying problem we encountered, that really needs fixing.
Have a great time in Lisbon, you won't regret it!